Add deref test

[tcmulcahy]

As requested in #57 I've created a failing regression test. This is just a copy of the existing test_path using the deref API variants. So it's also exercising set_deref/remove_deref/list_deref (those ones don't cause any problems yet).

cargo test output:

$ cargo test
    Finished test [unoptimized + debuginfo] target(s) in 0.02s
     Running unittests src/lib.rs (target/debug/deps/xattr-c5ae2acac45a8796)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/main.rs (target/debug/deps/main-1a74d7ef0eef6ac8)

running 5 tests
test test_missing ... ok
test test_fd ... ok
test test_multi ... ok
test test_path ... ok
test test_path_deref ... FAILED

failures:

---- test_path_deref stdout ----
thread 'test_path_deref' panicked at tests/main.rs:135:51:
called `Result::unwrap()` on an `Err` value: Os { code: 34, kind: Uncategorized, message: "Result too large" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_path_deref

test result: FAILED. 4 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--test main`

[Stebalien]

Looks like this has been fixed upstream. Thanks!