This came out of a conversation from CDRIVER-3964.

The C driver does not have a distinct helper, so operation_distinct in the unified test runner just creates a distinct command to send to the server. This does not actually test any C driver functionality, so we should really just skip tests that use distinct operations.

We may need to skip more spec tests with unsupported operations (such as collection enumeration variants) in the future, so having a specific function like "is_test_skipped" to explicitly enumerate skipped tests would be helpful.