[avalanche] Don't poll for txs below -blockmintxfee

Summary:
This makes sure we don't violate the -blockmintxfee configuration when using avalanche preconsensus.

Note this also adds some coverage for the -blockmintxfee option which was lacking before this diff.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_finalization

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18046