Rename wallet_accounts.py test
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.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D3624