[tests] move generate_wif_key to wallet_util.py

Summary:
generate_wif_key is a wallet utility function. Move
it from the EC key module to the wallet util module.

This is a backport of core#19239

Backport note: the revert commit is not relevant, we haven't backported core#19230

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9934