test_bitcoin: Avoid read/write to default datadir

Summary:
Partial backport of Core PR13663
https://github.com/bitcoin/bitcoin/pull/13663/commits/ea80b81e2e6d494a03177af50edef73f22820b3f

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4685