iotests: Basic tests for internal snapshots

We have a few test cases that include tests for corner case aspects of
internal snapshots, but nothing that tests that they actually function
as snapshots or that involves deleting a snapshot. Add a test for this
kind of basic internal snapshot functionality.

The error cases include a regression test for the crash we just fixed
with snapshot operations on inactive images.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231201142520.32255-4-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit bb6e2511eb48539b7dcbcb5f47772e156b9c45d1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>