Make test_compileall more robust by using -S to keep sys.path minimized.

Try this again, hopefully the right way this time.

Arfrever Taifersar Arahesis reported that test_compileall failed during Gentoo
install because it was tyring to write .pyc files to a read-only system
directory during test_no_args_compiles_path.  Having the tests call python
with -S should eliminate the system directories from the path.

Lib/test/test_compileall.py

@@ -126,13 +126,13 @@ class CommandLineTests(unittest.TestCase):
 
     def assertRunOK(self, *args, **env_vars):
         rc, out, err = script_helper.assert_python_ok(
-            '-m', 'compileall', *args, **env_vars)
+                        '-S', '-m', 'compileall', *args, **env_vars)
         self.assertEqual(b'', err)
         return out
 
     def assertRunNotOK(self, *args, **env_vars):
         rc, out, err = script_helper.assert_python_failure(
-                         '-m', 'compileall', *args, **env_vars)
+                        '-S', '-m', 'compileall', *args, **env_vars)
         return rc, out, err
 
     def assertCompiled(self, fn):