gh-54781: Move Lib/tkinter/test/test_ttk/ to Lib/test/test_ttk/ (#94070)

* Move Lib/tkinter/test/test_tkinter/ to Lib/test/test_tkinter/.
* Move Lib/tkinter/test/test_ttk/ to Lib/test/test_ttk/.
* Add Lib/test/test_ttk/__init__.py based on test_ttk_guionly.py.
* Add Lib/test/test_tkinter/__init__.py
* Remove old Lib/test/test_tk.py.
* Remove old Lib/test/test_ttk_guionly.py.
* Add __main__ sub-modules.
* Update imports and update references to rename files.

Lib/test/test_tkinter/test_loadtk.py

@@ -0,0 +1,46 @@
+import os
+import sys
+import unittest
+import test.support as test_support
+from test.support import os_helper
+from tkinter import Tcl, TclError
+
+test_support.requires('gui')
+
+class TkLoadTest(unittest.TestCase):
+
+    @unittest.skipIf('DISPLAY' not in os.environ, 'No $DISPLAY set.')
+    def testLoadTk(self):
+        tcl = Tcl()
+        self.assertRaises(TclError,tcl.winfo_geometry)
+        tcl.loadtk()
+        self.assertEqual('1x1+0+0', tcl.winfo_geometry())
+        tcl.destroy()
+
+    def testLoadTkFailure(self):
+        old_display = None
+        if sys.platform.startswith(('win', 'darwin', 'cygwin')):
+            # no failure possible on windows?
+
+            # XXX Maybe on tk older than 8.4.13 it would be possible,
+            # see tkinter.h.
+            return
+        with os_helper.EnvironmentVarGuard() as env:
+            if 'DISPLAY' in os.environ:
+                del env['DISPLAY']
+                # on some platforms, deleting environment variables
+                # doesn't actually carry through to the process level
+                # because they don't support unsetenv
+                # If that's the case, abort.
+                with os.popen('echo $DISPLAY') as pipe:
+                    display = pipe.read().strip()
+                if display:
+                    return
+
+            tcl = Tcl()
+            self.assertRaises(TclError, tcl.winfo_geometry)
+            self.assertRaises(TclError, tcl.loadtk)
+
+
+if __name__ == "__main__":
+    unittest.main()