Rename wallet_accounts.py test

Summary:
This is a separate commit because changing the test at the same time as
renaming it breaks git (and github) rename detection.

This is the finalization of the backport of Core PR11536 because D1986 was not the full thing.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3624