qa: Cache only chain and wallet for regtest datadir

Summary:
Backport of core PR12638
https://github.com/bitcoin/bitcoin/pull/12638/files

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3082