[tests] refactor interface_rest.py to avoid code repetition

Summary:

Also refactor txout index parsing and formatting.

Partial backport of core PR12766 (commit ad00fbe)
https://github.com/bitcoin/bitcoin/pull/12766/commits/ad00fbed3cfcfe7e921500179183589ac4aad419

Depends on D3767

Test Plan:

./test/functional/test_runner.py interface_rest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3768