Issue #21337: Add bare-bones Tix test

All this does is confirm that Tix is available on Windows, which should
always be the case (if _tkinter itself is available).

Lib/test/test_tix.py

+import unittest
+from test import support
+import sys
+
+# Skip this test if the _tkinter module wasn't built.
+_tkinter = support.import_module('_tkinter')
+
+# Skip test if tk cannot be initialized.
+support.requires('gui')
+
+from tkinter import tix, TclError
+
+
+class TestTix(unittest.TestCase):
+
+    def setUp(self):
+        try:
+            self.root = tix.Tk()
+        except TclError:
+            if sys.platform.startswith('win'):
+                self.fail('Tix should always be available on Windows')
+            self.skipTest('Tix not available')
+        else:
+            self.addCleanup(self.root.destroy)
+
+    def test_tix_available(self):
+        # this test is just here to make setUp run
+        pass
+
+
+if __name__ == '__main__':
+    unittest.main()