Previously it was looping, waiting for FINISHED|FAILED but was not
actually failing the test if the compose failed to build.
This adds a function to check the status of the compose and calls it
after each compose.
Beakerlib upstream can't do this yet, but might at some point:
https://github.com/beakerlib/beakerlib/issues/42
This is only enabled in combination with the `--sit` option of the
`test/check-*` scripts. It leaves the system in exacly the state it was
in when an assertion failed. Finishing the test run would run cleanup as
well (such as deleting created images). It also takes longer.
