Add blocknotify and walletnotify functional tests

Summary:

• [tests] Tidy up forknotify.py
• [tests] Add -blocknotify functional test
• [tests] Add -walletnotify functional test

This is a backport of Core PR10941

Depends on D1940

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1941