Remove pov-recovery race condition/Improve zombienet test (#2526)

The test was a bit flaky on CI. 

There was a race condition in the pov-recovery system. If the timing is
bad, it can happen that a block waits for a parent that is already
queued for import. The check if a block has children waiting happens
when we insert into the import queue. So we need to do an additional
check once we receive the import notification for the parent block.

Second issue is that `alice` was missing `--in-peers 0` and `--out-peers
0`, so alice was sometimes still fetching block via sync and the
assertion on the logs in zombienet would fail.

There is another potential issue that I saw once locally. We have a
failing pov-recovery queue that fails from time to time to check that
the retry mechanism does what it should. We now make sure that the same
candidate is never failed twice, so the tests become more predictable.