Improve post-test cleanup

The multi-region tests were not being diligent
about cleaning up their buckets / keys
and making sure that those deletes were synced.
This was causing the nuke_prefixed_buckets()
method to run into errors.
This patch adds more cleanup code and
validation of that cleanup within
the pertinent tests.

Signed-off-by: Joe Buck <jbbuck@gmail.com>