change bucket_fixup_test:fixup_test_/0 to wait for ring manager death

riak_core_ring_manager:stop/0 is async and races w/ the start of load_test_/0. see #590
Merge pull request #592 from basho/bugfix/jrw/590-bucket-fixup-test-cleanup

change bucket_fixup_test:fixup_test_/0 to wait for ring manager death

Reviewed-by: reiddraper