Issue #13890: Fix importlib case-sensitivity tests to not run on Windows.

Thanks to os.environ under Windows only updating the dict and not the
environment itself (as exposed by nt.environ), tests using
PYTHONCASEOK always fail. Now the tests are skipped when os.environ
does not do what is expected.

Lib/importlib/test/source/test_case_sensitivity.py

@@ -37,6 +37,9 @@ class CaseSensitivityTest(unittest.TestCase):
     def test_sensitive(self):
         with test_support.EnvironmentVarGuard() as env:
             env.unset('PYTHONCASEOK')
+            if b'PYTHONCASEOK' in _bootstrap._os.environ:
+                self.skipTest('os.environ changes not reflected in '
+                              '_os.environ')
             sensitive, insensitive = self.sensitivity_test()
             self.assertTrue(hasattr(sensitive, 'load_module'))
             self.assertIn(self.name, sensitive.get_filename(self.name))
@@ -45,6 +48,9 @@ class CaseSensitivityTest(unittest.TestCase):
     def test_insensitive(self):
         with test_support.EnvironmentVarGuard() as env:
             env.set('PYTHONCASEOK', '1')
+            if b'PYTHONCASEOK' not in _bootstrap._os.environ:
+                self.skipTest('os.environ changes not reflected in '
+                              '_os.environ')
             sensitive, insensitive = self.sensitivity_test()
             self.assertTrue(hasattr(sensitive, 'load_module'))
             self.assertIn(self.name, sensitive.get_filename(self.name))