Add a minimal unit test for Python/frozen.c. (#2995)

If the marshal or bytecode formats get changed, frozen.c needs to
be updated as well.  It can be easy to miss this step and not doing
so can cause test_importlib to crash in mysterious ways.  Add an
explict unit test to make it easier to track down the problem.

Lib/test/test_frozen.py

+"""Basic test of the frozen module (source is in Python/frozen.c)."""
+
+# The Python/frozen.c source code contains a marshalled Python module
+# and therefore depends on the marshal format as well as the bytecode
+# format.  If those formats have been changed then frozen.c needs to be
+# updated.
+#
+# The test_importlib also tests this module but because those tests
+# are much more complicated, it might be unclear why they are failing.
+# Invalid marshalled data in frozen.c could case the interpreter to
+# crash when __hello__ is imported.
+
+import sys
+import unittest
+from test.support import captured_stdout
+from importlib import util
+
+
+class TestFrozen(unittest.TestCase):
+    def test_frozen(self):
+        name = '__hello__'
+        if name in sys.modules:
+            del sys.modules[name]
+        with captured_stdout() as out:
+            import __hello__
+        self.assertEqual(out.getvalue(), 'Hello world!\n')
+
+
+if __name__ == '__main__':
+    unittest.main()