Python: Replace rest of from testlib import *

I think we should write our tests in a way that puts points-to in the
best condition to resolve calls. Although this specific change did not
change much, it should help set us up for success in the future 👍

python/ql/test/experimental/dataflow/coverage/classes.py

@@ -12,7 +12,7 @@ import sys
 import os
 
 sys.path.append(os.path.dirname(os.path.dirname((__file__))))
-from testlib import *
+from testlib import expects
 
 import asyncio