test: Fix combine_logs.py for windows

Summary:
The regex used to find the chain name in debug.log path did not work on Windows due to the different path separator.
This uses pathlib to make sure the path used for the regex search is as_posix()

This is a backport of Core PR16973

Test Plan:
I tested that it still works on my linux machine, with the same test as D7796

Intentionaly modify a functional test to make it fail (assert False), then run:

../test/functional/feature_bip68_sequence.py --configfile=./test/config.ini
../test/functional/combine_logs.py -c

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8150