[backport#15927] [tests] log thread names by default in functional tests

Summary:
[tests] Comment for why logging config is set as command-line args. (John Newbery)
[tests] log thread names by default in functional tests (John Newbery)

Pull request description:

More detailed logs are better

https://github.com/bitcoin/bitcoin/pull/15927/files

Backport of Core PR15927

Test Plan:

ninja
test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7082