Rename wallet functional test files to match bitcoin core
Summary: Per title. Depends on D1986
Test Plan:
./test/functional/test_runner.py wallet_*.py
Reviewers: #bitcoin_abc, deadalnix, Fabien
Reviewed By: #bitcoin_abc, deadalnix, Fabien
Subscribers: Fabien, deadalnix, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1992