mgr/volumes: Tests amended and added to ensure subvolume trash functionality

mgr/volumes: Tests amended and added to ensure subvolume trash functionality

Amended a few test cases to ensure created subvolumes and snaps
are removed, and trash stays empty at the end of the test.

Further added one test case for create errors in a retained
v2 subvolume, to ensure metadata is sane, and created incarnation
is not present.

Fixes: https://tracker.ceph.com/issues/47154
Signed-off-by: Shyamsundar Ranganathan <srangana@redhat.com>
(cherry picked from commit 88cffe6ea97733f2f0169362ffbde3e3827dc861)