test: further extend systemctl's sanity coverage
    
    Also, fix a race condition introduced by d16684fe13:
    ```
    [   16.904218] H testsuite-26.sh[394]: + systemd-run --unit failed.service /bin/false
    [   16.964783] H systemd[845]: failed.service: Executing: /bin/false
    [   16.965062] H systemd[1]: Started failed.service.
    [   16.965462] H testsuite-26.sh[844]: Running as unit: failed.service
    [   16.966390] H testsuite-26.sh[394]: + systemctl is-failed failed.service
    [   16.977970] H testsuite-26.sh[846]: active
    [   16.978403] H systemd[1]: failed.service: Main process exited, code=exited, status=1/FAILURE
    [   16.978478] H systemd[1]: failed.service: Failed with result 'exit-code'.
    ```
    
    (cherry picked from commit 23f3a6f5ff864fd26063c6c35fdaa6d85de566c7)