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.

diff --cc Lib/test/support.py
index be1c428c60f67ae550e0ff307611b8f0b174aacb,8db90a2a8cd7b943b3c2575f8881c46ac2e38f99..5b7cfc2d49d528b63377fe25daf26c4044a67ff1
--- 1/Lib/test/support.py
--- 2/Lib/test/support.py
+++ b/Lib/test/support.py
@@@ -39,12 -39,17 +39,17 @@@ except ModuleNotFoundError
  
  try:
      import zlib
 -except ImportError:
 +except ModuleNotFoundError:
      zlib = None
  
 -except ImportError:
+ try:
+     import gzip
++except ModuleNotFoundError:
+     gzip = None
+ 
  try:
      import bz2
 -except ImportError:
 +except ModuleNotFoundError:
      bz2 = None
  
  try:

diff --cc Lib/test/test_tarfile.py
index a69437c796d6d2378b4b8ff39326da1fe59d2a81,238175ff3c1c5bab6fc9598e9ce24869d5e697c5..f3cb464f57a23f5ae0224c547ffa5047231dcb55
--- 1/Lib/test/test_tarfile.py
--- 2/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@@ -14,16 -12,15 +12,15 @@@ from test import suppor
  # Check for our compression modules.
  try:
      import gzip
-     gzip.GzipFile
- except (ImportError, AttributeError):
 -except ImportError:
++except ModuleNotFoundError:
      gzip = None
  try:
      import bz2
--except ImportError:
++except ModuleNotFoundError:
      bz2 = None
  try:
      import lzma
--except ImportError:
++except ModuleNotFoundError:
      lzma = None
  
  def md5sum(data):