Add settings_tests

Summary:
Co-authored-by: James O'Beirne <james.obeirne@gmail.com>

This is a partial backport of Core PR15934 : https://github.com/bitcoin/bitcoin/pull/15934/commits/083c954b02a4e7d0708349eeaf3bac2b5947fb0e

Depends on D5903

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5904