[backport#17593][test] move wallet helper functions into test library

Summary:
https://github.com/bitcoin/bitcoin/pull/17593/commits/78e283e656bf1643944ffdb76185f3468eb25895

Depends on D7284

Concludes backport of Core PR17593

Test Plan:

ninja all check check-functional
./src/bench/bitcoin-bench

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7285