Fix plugins check

[tdivis]

Check for required_sane_plugins, called from is_skipped() in startTest was causing errors because not all plugins were enabled at that time (startTest methods of all plugins are called sequentially), so enable_test had to be moved to beforeTest plugin method.

This was revealed by failing test testproject.test.test_database.TestFixturesLoadedProperly.test_model_loaded, which now passes.