Make sure to filter warnings to errors

One test case after the transition from pytest.warns(None) is attempting
to check it raises no warnings, but as written the context manager
swallows all warnings. Set the filter to error to make certain they are
raised.

jenkinsapi_tests/unittests/test_build.py

@@ -277,6 +277,7 @@ def fake_get_data(cls, tree=None, params=None):
 
     with pytest.raises(Exception) as excinfo:
         with warnings.catch_warnings():
+            warnings.simplefilter("error")
             build.get_env_vars()
     assert "" == str(excinfo.value)