Make mprovements + change integration test name

[wszaranski]

Make improvements

• removed reduntant integration Makefile
• changed "integration" target to run only integration tests
  this should speed up CI tests because they wont run unit tests twice
  (in testrace and again in int target)
• added help target to list available targets to be used by user
• added target to download and build redis server
  server will be downloaded and build if it's not available
  when version in get_redis.sh will be changed new server will be
  downloaded and build automatically even if previous version was
  available locally

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Change integration test name to conformance

Integration tests test application code/module together with other
software. In case of miniredis there is no connection between miniredis
code and original redis. Vanilla Redis is used in tests only as source
of truth (basically as part of the test code not tested application).
Conformance test name should be more obvious in this case.

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NOTE: it's my personal opinion, that in case of miniredis, "integration" test is a little misleading name and "conformance" might be a better one. I don't have strong opinion on this, so if you prefer to keep old name, don't hesitate to drop this commit without any comments :)

[alicebob]

Hi! Thanks for the updates. I did not use all.

• I prefer to not have to configure my editor much, and expect make to do something sane. Which is often go test, so I keep running make doing just that. Same reason there's a Makefile in integration: then I can run my "run make" button in my editor and it'll do the right thing.
• the redis binary doesn't have to come from the local copy. If there's a redis-server in the path the tests will use that one. So you can download a binary and don't have to build from source.
• renaming integration to something more useful you're are completely right about, but it has been like this forever and it doesn't seem worth the trouble.