[tests] Rename p2p_* functional tests

Summary:
Rename only, except a minor update in doc/functional-tests.md

Partial backport of core PR11774 (commit 61b8f7f)

Depends on D2378

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2379