test: use fs namespace in dbwrapper unicodepath test

Summary:
This is a backport of Core PR18371

Depends on D8285

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8284