doc: Describe log files + consistent paths in test READMEs

Summary:
This is a backport of core#17177

Note: we use absolute paths (relative to the root of the project) because we store this documentation file in doc/ while BTC stores it in test/

Test Plan: proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9900