Test suite: enable garbage collection of failed transient units created by systemd-run

I did not enable this initially because it requires systemd v236 or newer, and back then we used to support running our test suite on Stretch (which has v232).

Since #17842 (closed) we require Buster or newer to run our test suite, and Buster has systemd v241, so we can now enable this.

Without --collect, when such a transient unit fails, it would be left behind and the next test suite run would fail to create a transient unit with the same name again, until the developer figures out how to clean things up themselves — which, while entirely feasible, is not exactly trivial.