doc: Describe log files + consistent paths in test READMEs
doc: Describe log files + consistent paths in test READMEs

Summary:
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/17177 | 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

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