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.
* Replace ImportError by  ModuleNotFoundError.

and some other minor enhancements.

Lib/test/support.py

@@ -42,6 +42,11 @@ try:
 except ModuleNotFoundError:
     zlib = None
 
+try:
+    import gzip
+except ModuleNotFoundError:
+    gzip = None
+
 try:
     import bz2
 except ModuleNotFoundError:
@@ -71,7 +76,7 @@ __all__ = [
     "TestHandler", "Matcher", "can_symlink", "skip_unless_symlink",
     "skip_unless_xattr", "import_fresh_module", "requires_zlib",
     "PIPE_MAX_SIZE", "failfast", "anticipate_failure", "run_with_tz",
-    "requires_bz2", "requires_lzma", "suppress_crash_popup",
+    "requires_gzip", "requires_bz2", "requires_lzma", "suppress_crash_popup",
     ]
 
 class Error(Exception):
@@ -590,6 +595,8 @@ requires_IEEE_754 = unittest.skipUnless(
 
 requires_zlib = unittest.skipUnless(zlib, 'requires zlib')
 
+requires_gzip = unittest.skipUnless(gzip, 'requires gzip')
+
 requires_bz2 = unittest.skipUnless(bz2, 'requires bz2')
 
 requires_lzma = unittest.skipUnless(lzma, 'requires lzma')