Add test case and release notes for auto finalization

Summary:
As per title.

Depends on D2102

Test Plan:

make check
./test/functional/test_runner.py abc-finalize-block

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2106