Actually run assumevalid.py.
Summary:
assumevalid was merged as part of PR 9484, but was not added to the
test_runner, so is not run even as part of the extended tests.
This commit adds assumevalid to the list of tests in test_runner. It
also clarifies the code in assumevalid considerably.
This is a backport of core's PR10073
Test Plan:
make check ./test/functional/test_runner.py assumevalid.py
Reviewers: #bitcoin_abc, schancel
Reviewed By: #bitcoin_abc, schancel
Differential Revision: https://reviews.bitcoinabc.org/D974