use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_unittest.py b/Lib/test/test_unittest.py
index e33f31e..95febe1 100644
--- a/Lib/test/test_unittest.py
+++ b/Lib/test/test_unittest.py
@@ -636,7 +636,7 @@
             self.assertEqual(list(suite), [])
 
             # audioop should now be loaded, thanks to loadTestsFromName()
-            self.assertTrue(module_name in sys.modules)
+            self.assertIn(module_name, sys.modules)
         finally:
             if module_name in sys.modules:
                 del sys.modules[module_name]
@@ -1024,7 +1024,7 @@
             self.assertEqual(list(suite), [unittest.TestSuite()])
 
             # audioop should now be loaded, thanks to loadTestsFromName()
-            self.assertTrue(module_name in sys.modules)
+            self.assertIn(module_name, sys.modules)
         finally:
             if module_name in sys.modules:
                 del sys.modules[module_name]