test_rbd: close image before removing it

This error was masked before by watch notify not differentiating
between watches from the same client with different cookies.
Reopen the image at the end of this test so teardown works.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
qa: enable watch-notify dependent test

This works now that watch-notify has been reworked a bit.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
test_rbd: move flatten tests back into TestClone

They need the same setup, and it's easy enough to run specific
subtests. Making them a separate subclass accidentally duplicated
tests from TestClone.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge remote-tracking branch 'gh/wip-4252'