(PUP-2298) protect transaction spec test against new exceptions

The new perform_pre_run_checks method in the transaction loops over
all resources in the catalog. This is prone to failures if any nil references
end up in the catalog. This can happen during the addition of resources
to the catalog under test conditions, because the mock resources are not
sufficiently contained.

To work around this effect, stub the very #container_of method in the mock
catalog so that all resources will have an apparent container.