testcase: improve leaked partitions detection and cleanup

- We should only try to remove leaked partitions if we detected some,
  otherwise it may produce a strange message "unable to delete leaked
  partitions".
- If some supervisor configuration is still present after partition
  deletion, they must be removed not to block next test and the current
  test must be marked failed.
7 jobs for <span class="ref-name">fix/standalone_fixes_plus_save_logs</span> in 0 seconds
Status Job ID Name Coverage
  External
running SLAPOS-SR-TEST-JEROME-WIP

6840:27:12

canceled SLAPOS-SR-TEST-JEROME-WIP

03:43:44

canceled SLAPOS-SR-TEST-JEROME-WIP

failed SLAPOS-SR-TEST-JEROME-WIP

04:07:54

canceled SLAPOS-SR-TEST-JEROME-WIP

failed SLAPOS-SR-TEST-JEROME-WIP

06:27:43

failed SLAPOS-SR-TEST-JEROME-WIP

09:23:32