test: Add basic testing for wallet groups

Summary:
This is a partial backport of Core PR12257 : https://github.com/bitcoin/bitcoin/pull/12257/commits/0128121101fb3ee82f3abd3973a967a4226ffe0e

Depends on D3394

Also include the test for a later bugfix:

• [qa] Add test for too-large wallet output groups

This is extracted from Core PR13805 : https://github.com/bitcoin/bitcoin/pull/13805/commits/a13647b8bd667ca58d8e82682c1d46555dce42c9

Test Plan:

test/functional/test_runner.py wallet_groups

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3395