Skip some tests that require a subinterpreter launched with -E or -I when the

interpreter under test is being run in an environment that requires the use of
environment variables such as PYTHONHOME in order to function at all.

Adds a test.script_helper.interpreter_requires_environment() function
to be used with @unittest.skipIf on stdlib test methods requiring this.

Lib/test/test_script_helper.py

@@ -1,7 +1,10 @@
 """Unittests for test.script_helper.  Who tests the test helper?"""
 
+import subprocess
+import sys
 from test import script_helper
 import unittest
+from unittest import mock
 
 
 class TestScriptHelper(unittest.TestCase):
@@ -31,5 +34,43 @@ class TestScriptHelper(unittest.TestCase):
                       msg='unexpected command line.')
 
 
+class TestScriptHelperEnvironment(unittest.TestCase):
+    """Code coverage for interpreter_requires_environment()."""
+
+    def setUp(self):
+        self.assertTrue(
+                hasattr(script_helper, '__cached_interp_requires_environment'))
+        # Reset the private cached state.
+        script_helper.__dict__['__cached_interp_requires_environment'] = None
+
+    def tearDown(self):
+        # Reset the private cached state.
+        script_helper.__dict__['__cached_interp_requires_environment'] = None
+
+    @mock.patch('subprocess.check_call')
+    def test_interpreter_requires_environment_true(self, mock_check_call):
+        mock_check_call.side_effect = subprocess.CalledProcessError('', '')
+        self.assertTrue(script_helper.interpreter_requires_environment())
+        self.assertTrue(script_helper.interpreter_requires_environment())
+        self.assertEqual(1, mock_check_call.call_count)
+
+    @mock.patch('subprocess.check_call')
+    def test_interpreter_requires_environment_false(self, mock_check_call):
+        # The mocked subprocess.check_call fakes a no-error process.
+        script_helper.interpreter_requires_environment()
+        self.assertFalse(script_helper.interpreter_requires_environment())
+        self.assertEqual(1, mock_check_call.call_count)
+
+    @mock.patch('subprocess.check_call')
+    def test_interpreter_requires_environment_details(self, mock_check_call):
+        script_helper.interpreter_requires_environment()
+        self.assertFalse(script_helper.interpreter_requires_environment())
+        self.assertFalse(script_helper.interpreter_requires_environment())
+        self.assertEqual(1, mock_check_call.call_count)
+        check_call_command = mock_check_call.call_args[0][0]
+        self.assertEqual(sys.executable, check_call_command[0])
+        self.assertIn('-E', check_call_command)
+
+
 if __name__ == '__main__':
     unittest.main()