test: use non-blocking systemctl calls in testsuite.service

"systemctl poweroff" called from testsuite.service will cause this unit
itself to stop. To avoid deadlock, the call must not be synchronous.
test: factor out testsuite.target, end.service

Tests can use the same testsuite.target.
Add end.service to call poweroff instead of doing it from ExecStopPost
where it may be skipped on failure of ExecStart.
test: add test for jobs

Add a test case for job merging with --ignore-dependencies.

test.sh is copied from TEST-01-*, only lightly modified (this
should be refactored better in the future).
test-jobs.sh is the core of this test.
job: fix merging with --ignore-dependencies

This fixes a bug where a job with --ignore-dependencies would wait for
other jobs because it merged into a previously queued job.