get_user_groups() originally returned a dictionary

This appears to have been accidentally changed in #3563 (Oct 2022) when we ported a number of queries to use QueryProcessor.

The docstring still indicates it should return a dictionary.

is_build_owner also assumes it should return a dictionary

test_policy_tests.py mistakenly mocks its return value as a list of ints (in one test), which matches neither the original behavior nor the current.

get_user_groups isn't used that much in the code, but in #3855 we might want to use it more

ah, mea culpa - I've not look to history.

:thumbsup:

Commit dc3dfc99 fixes this pull-request