Restore default testRunner argument in unittest.main to None. Issue 6177
diff --git a/Lib/test/test_unittest.py b/Lib/test/test_unittest.py
index 950b2ca..ecf3162 100644
--- a/Lib/test/test_unittest.py
+++ b/Lib/test/test_unittest.py
@@ -3325,6 +3325,14 @@
         self.assertEqual(program.verbosity, 2)
 
 
+    def testTestProgram_testRunnerArgument(self):
+        program = object.__new__(TestProgram)
+        program.parseArgs = lambda _: None
+        program.runTests = lambda: None
+        program.__init__(testRunner=None)
+        self.assertEqual(program.testRunner, unittest.TextTestRunner)
+
+
     class FooBar(unittest.TestCase):
         def testPass(self):
             assert True