tests: Fix cross-test interaction
tests: Fix cross-test interaction

Previously, we always needed to retry test_statsd_set_prefix_deprecation.
This was because the warning would be triggered in
test_get_logger_statsd_client_non_defaults and recorded in the
module-level warnings registry. Now, explicitly clear the warning
registry. From the docs [0]:

> One thing to be aware of is that if a warning has already been raised
> because of a once/default rule, then no matter what filters are set
> the warning will not be seen again unless the warnings registry
> related to the warning has been cleared.

[0] https://docs.python.org/3/library/warnings.html#testing-warnings

Change-Id: Icf4b381dcc04d04b5401e5ed3f43df049c1dd2b4

tipabu/swift,tipabu/swift,tipabu/swift,tipabu/swift,openstack/swift,openstack/swift,openstack/swift,openstack/swift