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.