Issue #18223: Refactor test_tarfile.

* Use mixins for generating tests for different compression types.
* Make test_tarfile discoverable.
* Use more special tests (i.e. assertEqual, assertIs) instead of assertTrue.
* Add explicit test skips instead of reporting skipped tests as passed.
* Wrap long lines.
* Correct a comment for test_hardlink_extraction1.
* Add support.requires_gzip.

and some other minor enhancements.