Issue #18441: Make test.support.requires('gui') skip when it should.

Lib/test/test_idle.py

-# Skip test if _tkinter or _thread wasn't built or idlelib was deleted.
-from test.support import import_module
-import_module('tkinter')
+import unittest
+from test import support
+from test.support import import_module, use_resources
+
+# Skip test if _thread or _tkinter wasn't built or idlelib was deleted.
 import_module('threading')  # imported by PyShell, imports _thread
+tk = import_module('tkinter')  # imports _tkinter
 idletest = import_module('idlelib.idle_test')
 
+# If buildbot improperly sets gui resource (#18365, #18441), remove it
+# so requires('gui') tests are skipped while non-gui tests still run.
+# If there is a problem with Macs, see #18441, msg 193805
+if use_resources and 'gui' in use_resources:
+    try:
+        root = tk.Tk()
+        root.destroy()
+    except tk.TclError:
+        while 'gui' in use_resources:
+            use_resources.remove('gui')
+
 # Without test_main present, regrtest.runtest_inner (line1219) calls
 # unittest.TestLoader().loadTestsFromModule(this_module) which calls
 # load_tests() if it finds it. (Unittest.main does the same.)
@@ -13,7 +27,5 @@ if __name__ == '__main__':
     # Until unittest supports resources, we emulate regrtest's -ugui
     # so loaded tests run the same as if textually present here.
     # If any Idle test ever needs another resource, add it to the list.
-    from test import support
     support.use_resources = ['gui']  # use_resources is initially None
-    import unittest
     unittest.main(verbosity=2, exit=False)