bpo-40280: Skip more tests/features that don't apply to Emscripten (GH-31791)

- fd inheritance can't be modified because Emscripten doesn't support subprocesses anyway.
- setpriority always fails
- geteuid no longer causes problems with latest emsdk
- umask is a stub
- geteuid / getuid always return 0, but process cannot chown to random uid.

Lib/test/test_tarfile.py

@@ -1498,6 +1498,7 @@ class StreamWriteTest(WriteTestBase, unittest.TestCase):
 
     @unittest.skipUnless(sys.platform != "win32" and hasattr(os, "umask"),
                          "Missing umask implementation")
+    @unittest.skipIf(support.is_emscripten, "Emscripten's umask is a stub.")
     def test_file_mode(self):
         # Test for issue #8464: Create files with correct
         # permissions.