test, refactor: add default_wallet_name and wallet_data_filename variables

Summary:
No changes in behavior

This is a backport of core#20034 [1/2]
https://github.com/bitcoin/bitcoin/pull/20034/commits/ed3acda33b75d1b546ee696a63def239bcdd62de

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10421