Merge #12766: [Tests] Tidy up REST interface functional tests

55efc1f [tests] simplify binary and hex response parsing in interface_rest.py (Roman Zeyde)
ade5964 [tests] only use 2 nodes in interface_rest.py (John Newbery)
ad00fbe [tests] refactor interface_rest.py to avoid code repetition (John Newbery)
7a3181a [tests] Make json request building more consistent in interface_rest.py (John Newbery)
3fd4490 [tests] improve logging and documentation in interface_rest.py (John Newbery)
abf190e [tests] fix flake8 warnings in interface_rest.py test (John Newbery)

Pull request description:

  Following the comment at bitcoin/bitcoin#12717 (review).

Tree-SHA512: b55560f0d8f3069584f5a2398285483a0a23514b2b2bd2c1ced2db1cb30dc24f60f720d0fa4de30259f7918d3178d94680ae9321649544d1d04d687a2e672559

Author: laanwj