tests: have combine_logs default to most recent test dir

Summary:
Have combine_logs.py default to the most recent test directory if no argument is provided.
Backport of Core PR14683

Test Plan:

ninja
../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/D7795