test: Always shutdown on test success in mkosi

test: Always shutdown on test success in mkosi

When we want to get an interactive shell in a test that fails because
of a race condition, we might need to run the test a few times with
--repeat before it fails. However, currently, when -i is used, the VM
needs to be shut down manually each time before the next run can start.

Let's always shut down the VM if the test succeeds so that --repeat can
be used with -i to run the test until it fails and then get an interactive
shell in the VM.