Check that all test scripts in test/functional are being run

Summary:
This commit checks that all of the python files in the test/functional
directory are listed in test_runner.py.

This backports core's PR10096 and improves the situation on T163

Test Plan:
Remove one test from the test_runner.py

./test/functional/test_runner.py

Check that is gives me the following error:

The following scripts are not being run:['scriptname.py']
Chec the test list in test_runner.py

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D995