IDLE - fix module browser test (GH-100647)

(cherry picked from commit 1f6c87c)

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: miss-islington

Lib/idlelib/idle_test/test_browser.py

@@ -170,8 +170,7 @@ def test_ondoubleclick(self, fopen):
 
         with mock.patch('os.path.exists', return_value=True):
             mbt.OnDoubleClick()
-            fopen.assert_called()
-            fopen.called_with(fname)
+            fopen.assert_called_once_with(fname)
 
 
 class ChildBrowserTreeItemTest(unittest.TestCase):