plugin_test.py: Don't require pool with re-size all the time

A number of test arrays don't support re-sizable volumes.  That
however shouldn't prevent us from testing volume create/delete/
mask/unmask etc.  Only require a pool that allows re-sizeable
volumes when testing the ability to re-size volumes.

Signed-off-by: Tony Asleson <tasleson@redhat.com>