Address review: use ensure_lazy_imports, drop redundant flags test

Per review feedback on GH-154908:

- Rewrite test_compression_wrappers_not_imported_by_shutil on top of
  test.support.import_helper.ensure_lazy_imports instead of hand-rolling
  an assert_python_ok subprocess.
- Remove test_compression_supported_flags: since each wrapper imports its
  extension module at the top level, the assertion is near-tautological,
  and the zlib subtest compared importing zlib with itself.

Author: maxday

Lib/test/test_shutil.py

@@ -10,7 +10,6 @@
 import os.path
 import errno
 import functools
-import importlib
 import socket
 import subprocess
 import random
@@ -33,7 +32,7 @@
 from test import support
 from test.support import os_helper, socket_helper
 from test.support.os_helper import TESTFN, FakePath
-from test.support.script_helper import assert_python_ok
+from test.support.import_helper import ensure_lazy_imports
 
 TESTFN2 = TESTFN + "2"
 TESTFN_SRC = TESTFN + "_SRC"
@@ -2323,39 +2322,13 @@ def _boo(filename, extract_dir, extra):
         unregister_unpack_format('Boo2')
         self.assertEqual(get_unpack_formats(), formats)
 
-    def test_compression_supported_flags(self):
-        # shutil determines compression support by probing the extension
-        # modules (_bz2, _lzma, _zstd) rather than importing the pure Python
-        # wrappers.  The answer must match what importing the wrapper does,
-        # including on builds where the extension is missing or fails to load.
-        for wrapper, supported in (
-            ('zlib', shutil._ZLIB_SUPPORTED),
-            ('bz2', shutil._BZ2_SUPPORTED),
-            ('lzma', shutil._LZMA_SUPPORTED),
-            ('compression.zstd', shutil._ZSTD_SUPPORTED),
-        ):
-            with self.subTest(wrapper=wrapper):
-                try:
-                    importlib.import_module(wrapper)
-                except ImportError:
-                    importable = False
-                else:
-                    importable = True
-                self.assertEqual(supported, importable)
-
     def test_compression_wrappers_not_imported_by_shutil(self):
-        # Importing shutil must not pull in the compression wrappers: they are
-        # only needed once an archive is actually created or extracted, and
-        # importing them measurably slows down every process that uses shutil.
-        wrappers = ('bz2', 'lzma', 'compression', 'compression.zstd')
-        script = (
-            'import sys, shutil; '
-            f'print([m for m in {wrappers!r} if m in sys.modules])'
-        )
-        # -I so that a sitecustomize/usercustomize importing one of these
-        # cannot make the test fail spuriously.
-        rc, stdout, stderr = assert_python_ok('-I', '-c', script)
-        self.assertEqual(stdout.decode().strip(), '[]', stderr)
+        # gh-154904: Importing shutil must not pull in the compression
+        # wrappers: they are only needed once an archive is actually created
+        # or extracted, and importing them measurably slows down every
+        # process that uses shutil.
+        ensure_lazy_imports("shutil",
+                            {"bz2", "lzma", "compression", "compression.zstd"})
 
 
 class TestMisc(BaseTest, unittest.TestCase):