[tests] Make json request building more consistent in interface_rest.py

Summary:
Partial backport of core PR12766 (commit 7a3181a)
https://github.com/bitcoin/bitcoin/pull/12766/commits/7a3181a7676c822b2a369c745e0c45b0347c78e1

Depends on D3766

Test Plan:

./test/functional/test_runner.py interface_rest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3767