[avalanche] Fix a couple tests flakiness due to stalled txs

Summary:
Now that stalled txs are discarded it causes a couple tests to fail sometimes due to missing txs:

• For the chronik_ws test, we simply bump the stale thresold to a very high value to avoid this situation.
• For the abc_mining_avalanche_preconsensus test, make sure the non-preconsensus node has all the txs in its mempool before finalizing/invlidating any of them. It also requires to finalize/invalidate multiple txs at once instead of one by one to avoid intermediate states where some txs are finalized while their ancestors are not due to missing slots. As a consequence a small change in the def can_find_inv_in_poll is needed that could be leveraged in other tests as well. This also makes the test faster (stalling takes longer than finalizing/invlidating).

Test Plan:

./test/functional/test_runner.py --repeat 100 chronik_ws
./test/functional/test_runner.py --repeat 100 abc_mining_avalanche_preconsensus
ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18700