Add new tests for TagManager.find().

- test_find_returns_empty_when_no_tags_in_db
- test_find_returns_empty_when_user_matches_but_tag_name_doesnt
- test_find_returns_empty_when_tag_name_matches_but_user_doesnt
- test_find_returns_empty_when_search_string_is_not_a_prefix