Issue #27918# test.resource.is_gui_available no longer flashes tk window.

Also, don't run it if 'gui' is not requested.  Patch by Xiang Zhang.

Lib/test/test_support.py

@@ -331,6 +331,7 @@ def _is_gui_available():
         try:
             from Tkinter import Tk
             root = Tk()
+            root.withdraw()
             root.update()
             root.destroy()
         except Exception as e:
@@ -355,12 +356,12 @@ def is_resource_enabled(resource):
 
 def requires(resource, msg=None):
     """Raise ResourceDenied if the specified resource is not available."""
-    if resource == 'gui' and not _is_gui_available():
-        raise ResourceDenied(_is_gui_available.reason)
     if not is_resource_enabled(resource):
         if msg is None:
             msg = "Use of the `%s' resource not enabled" % resource
         raise ResourceDenied(msg)
+    if resource == 'gui' and not _is_gui_available():
+        raise ResourceDenied(_is_gui_available.reason)
 
 def requires_mac_ver(*min_version):
     """Decorator raising SkipTest if the OS is Mac OS X and the OS X