Fix FindLibraryTest by making it platform-dependent

[soonho-tri]

The original code was added by #7439.
See https://drake-jenkins.csail.mit.edu/view/Continuous%20Production/job/mac-sierra-clang-bazel-continuous-release/271/consoleText.

This PR merely changes the test to make CI happy. @fbudin69500, please check if the original test failure indicates an issue in LoadedLibraryPath.

/cc @jamiesnape, @sammy-tri, @jwnimmer-tri (reviewers of #7439)
/cc @ggould-tri (buildcop)

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This change is 

[soonho-tri]

@drake-jenkins-bot mac-sierra-clang-bazel-experimental-everything please
@drake-jenkins-bot mac-highsierra-clang-bazel-experimental please

[soonho-tri]

+@fbudin69500 for feature-review, please.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[fbudin69500]

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[fbudin69500]

Thanks for solving this issue. I'll double check that there is not a problem in the function that is tested, but this seems like a very simple fix that makes sense. The function LoadedLibraryPath() is typically used to check where libdrake.so is, but in this simple test, I didn't want to add complex dependencies to libdrake.so, so I settled in using libgflags.so which was used by this test. I forgot to verify that its name was the same on all platforms...
As a side note, example tests verify that this function works with libdrake.so, so I am fairly confident that that function works.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[soonho-tri]

+@EricCousineau-TRI for platform-review, please.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[EricCousineau-TRI]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

  // loaded by the process.
  optional<string> library_path =
    LoadedLibraryPath("libexternal_Scom_Ugithub_Ugflags_Ugflags_Slibgflags.so");

BTW It was a little hard for me to figure out at what point in the chain libglfags is effectively linked in as a shared rather than static library.

Could you either (a) manually create a test lib_is_real.so (using drake_cc_binary) to ensure that this test is not dependent on how we link libgflags (as it could change with something like #7616), or (b) document how libgflags is being dynamically linked, and under what conditions it would not be dynamically linked?

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Comments from Reviewable

[EricCousineau-TRI]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, EricCousineau-TRI wrote…

BTW It was a little hard for me to figure out at what point in the chain libglfags is effectively linked in as a shared rather than static library.

Could you either (a) manually create a test lib_is_real.so (using drake_cc_binary) to ensure that this test is not dependent on how we link libgflags (as it could change with something like #7616), or (b) document how libgflags is being dynamically linked, and under what conditions it would not be dynamically linked?

Sorry, promoting this to blocking rather than BTW.

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Comments from Reviewable

[jamiesnape]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, EricCousineau-TRI wrote…

Sorry, promoting this to blocking rather than BTW.

Can that be a follow up so that we get the Mac builds blue again?

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Comments from Reviewable

[EricCousineau-TRI]

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Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, jamiesnape (Jamie Snape) wrote…

Can that be a follow up so that we get the Mac builds blue again?

Yup.

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Comments from Reviewable

[soonho-tri]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Yup.

@fbudin69500 , could you open an issue to keep track of the items that @EricCousineau-TRI suggested?

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Comments from Reviewable

[EricCousineau-TRI]

common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, soonho-tri (Soonho Kong) wrote…

@fbudin69500 , could you open an issue to keep track of the items that @EricCousineau-TRI suggested?

I'll go ahead and do this; it should take about 10min (fingers crossed).

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Comments from Reviewable

[soonho-tri]

Review status: 0 of 1 files reviewed at latest revision, all discussions resolved, all commit checks successful.

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common/test/find_loaded_library_test.cc, line 16 at r1 (raw file):

Previously, EricCousineau-TRI wrote…

I'll go ahead and do this; it should take about 10min (fingers crossed).

Thank you. FYI, just merged this PR into master. I expect the mac CI will turn to blue!

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Comments from Reviewable