[RelDB] Fix and cleanup test framework

[luisa-beerboom]

• system/base.py:
  • in tearDown function: Investigate if smth like ShutdownService need to be called again.
  • write using events in set_committee_management_level, so the committee manager_ids don't need to be loaded and re-calculated all the time.
• f.E. poll/test_delete.py and poll/test_reset.py: Performance tests seem not to work, perhaps the ones in other files don't either. They should be fixed or replaced with a different approach.

[vkrasnovyd]

• f.E. poll/test_delete.py and poll/test_reset.py: Performance tests seem not to work, perhaps the ones in other files don't either. They should be fixed or replaced with a different approach.

All poll and vote tests are getting deleted in #3377 along with the corresponding actions. Poll tests are implemented in the new vote service.

[luisa-beerboom]

All poll and vote tests are getting deleted in #3377 along with the corresponding actions. Poll tests are implemented in the new vote service.

The other performance tests should still be looked at.