look into failing roundtrip tests on macos

[BurntSushi]

CI build failure: https://github.com/BurntSushi/rust-snappy/runs/2751122501?check_suite_focus=true

The failing tests are tests that check that this library generates the same compressed output as the reference Snappy C++ implementation. So a failing test here doesn't necessarily indicate a correctness issue, but that perhaps the reference implementation changed or got some improvements that we should investigate.

My next steps would probably be:

• Compare the versions on macOS and Linux CI. Are they the same? If so, try to understand why the failures might be happening on macOS and not Linux. If they aren't the same, try reproducing the problem on Linux with the same Snappy version used on macOS.
• Review the commit log for the reference Snappy C++ implementation and see if there are any relevant recentish changes.

For now, I've disabled running the cpp roundtrip tests on macOS in CI.

[Icelk]

This also occurs on Linux x64:

rust-snappy/test ❯ cargo test --all-features

...

failures:
    tests::qc_cmpcpp
    tests::random2::cmpcpp
    tests::random3::cmpcpp
    tests::random4::cmpcpp