[tests] Rename feature_* functional tests.
Summary:
Rename only, except a minor update in doc/functional-tests.md
Partial backport of core PR11774 (commit ca6523d)
Depends on D2384
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/D2364