[tests] move wallet util functions to wallet_util.py

Summary:
Adds a new wallet_util.py module and moves generic helper functions
there:

• get_key
• get_multisig
• test_address

This is a partial backport of Core PR15108 : https://github.com/bitcoin/bitcoin/pull/15108/commits/2d5f1ea2e3f9629d313f1d2aff94a15ef8e1128a

Depends on D6186 and D6185

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6187