Explicitly pytest.skip any tests that depend on optional dependencies

[dstansby]

As a quick fix in #637, some tests for the CRC32C codec are only ever defined if crc32c is installed. If crc32c isn't installed, these tests are silently not defined or reported to pytest.

In order to report that these tests are being skipped to pytest, it would be much better to mark them with a pytest skip mark, so when the dependency isn't installed these skipped tests get reported to pytest.