test-loop-block: run in qemu

test-loop-block needs to run in qemu, so we are currently not
testing it in the CI. Run it by itself in a separate job from
TEST-02-UNITTESTS to avoid slowing that suite down.

Fixes https://github.com/systemd/systemd/issues/19966

Disable it in the bionic-* CI for now, as it's affected by
the same uevent ordering issue as TEST-50-DISSECT which makes
it flaky.