Add assertion in test

Author: neonene

Lib/test/datetimetester.py

@@ -7285,6 +7285,7 @@ def gen():
                     assert not sys.modules
                     td = _datetime.timedelta(days=1)  # crash
                     assert td.days == 1
+                    assert not sys.modules
 
             it = gen()
             next(it)