test_util: merge load_fixture_and_fetch() into TestBase method The middle-term goal is to make TestBase repo_path and wc_path private, so they can be changed for every load call. This is not required to use nosetests multiprocess facility as the fixtures create temporary directories but it makes things much clearer and avoid weird cases where a repository was loaded several times at the same location in a single test (cf test_startrev). That way we will be more confident the tests can be parallelized. The long term goal is to make hgsubversion compatible with nosetests --processes option.