[#63207274] Reinstate bug test

The VM orchestrator fails silently when any part of the customization fails - the user is not warned that only some of their customizations have been applied.

There was an integration test that highlighted this bug. It was removed, and Sneha was planning to fix it and write a test to demonstrate it was fixed, but she was then off sick and then left the team, leaving the integration test deleted but no corresponding unit test added and the bug still present.

This PR reinstates the test and also, because it relies on them, some other improvements to the tests. Adding this test back in makes the full integration tests take ~2 minutes longer - about 11 mins instead of 9 mins - these tests are run only on CI not locally (i.e. they are not in `integration:quick`) so I think it's not too big a cost in order to have documentation of the bug.