[doc] Add suggestions in functional test documentation for improving debugging iteration speed

Summary:
It's common to iterate on tests many times before you get them right.
Save minutes of time and sanity with these tips.

Test Plan:

Read and try for yourself.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12893