piuparts: Properly discard the URL for testing

The PiupartsPolicy does not use the report URL for testing, so we do
not need to store it in memory.  Unfortunately, the logic was broken
and the discard did not happen.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/policies/policy.py

@@ -613,7 +613,7 @@ class PiupartsPolicy(BasePolicy):
             item = next(iter(suite_data.values()))
             state, _, url = item
             if not keep_url:
-                keep_url = None
+                url = None
             summary[source] = (state, url)
 
         return summary