Fix sources of flackiness in p2p-fullblocktest.py

Summary: Some of the test end up being dependent on transaction order (base on ids) and therefore flacky.

Test Plan:

./test/functional/test_runner.py p2p-fullblocktest.py

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2090