core: Add tests to StorageHandlingCommandBaseTest

Since the removal of DC type it's important to have test to test the logic
of whether attachment of domains to a pool is allowed, this patch adds tests
for the StorageHandlingCommandBase which holds most of the logic that
determines attachment is allowed

Change-Id: I97e45957f7ff0fe4c9e7d88e0d8dbab7e874c79b
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Relates-To: https://bugzilla.redhat.com/1038053