[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 [[https://github.com/bitcoin/bitcoin/pull/15927 | PR15927]]