Tidy up plugins in ui config tests
Tidy up plugins in ui config tests

Some of these tests load plugins using beets' normal plugin loader, but
didn't call unload_plugins to tidy up afterwards. This led to any future
plugin loads being ignored until the next unload_plugins call.

This commit changes the config tests so that we always call load_plugins
on setup (to store the default beets state) and unload_plugins on
teardown (to restore the previously stored state).

beetbox/beets,beetbox/beets,beetbox/beets,beetbox/beets