test: Mockwallet

Summary:

This introduces a minimalistic test wallet, which can be used as a drop in replacement for the Bitcoin Core wallet to create dummy transactions with a given fee rate.

This is a backport of core#19800

It is squashed with the first bugfix commit of core#19922:
https://github.com/bitcoin/bitcoin/pull/19922/commits/fa65a11d0c9a34ff7f4cc4efd53367794e751749

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10199