librbd: Refactor librbd unit tests to use test fixtures

Current librbd unit tests create and destroy pools for each
test.  By migrating to a test fixture, the same pool(s) are
now used for all tests within a test case.  This speeds up
the runtime of the librbd tests.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2873 from ceph/wip-rbd-test-fixtures

librbd: Refactor librbd unit tests to use test fixtures

Reviewed-by: Josh Durgin <jdurgin@redhat.com>