test: Explain why test logging should be used
test: Explain why test logging should be used

Summary: This is a backport of [[https://github.com/bitcoin/bitcoin/pull/18305 | core#18305]]

Test Plan: Proofreading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9915

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