Feature #36745: mgr/dashboard: Increase frontend E2E test suite
mgr/dashboard: fix notifications E2E tests
Fixes to avoid errors when testing notifications:
- Deleting a pool (after checking the running task is shown)
while the running task is not finished yet (pool update).
- Clear all notifications and checking there are no notifications
only once (when in the meantime another extra notification can appear, i.e.
I faced this when running these tests locally).