tests: various fixes * remove journal flushing (systemd-journal-flush.service runs journalctl --flush on boot) * use sh -c and PATH instead of @SYSTEMCTL@ expansion * remove unnecessary semicolons etc

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.