Part of the upcoming refactoring will change how broken chains
are detected; it makes sense to test that this works.  In
particular, test the just-fixed infinite loop detection bug.
Also, make sure that detection of directories is sane.

* tests/virstoragetest.c (testStorageChain): Enhance test.
(mymain): Add more tests.
(testCleanupImages, testPrepImages): Populate a directory.
Signed-off-by: NEric Blake <eblake@redhat.com>