Align unit test names with their filenames

Summary:
This consistency makes it easier to execute specific tests individually
because you know that using their base filename will work with
the --run_test=<testname> option of the runner.

This easily allows running a selection of focused test based on matching patterns in their filenames.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D350