Add regtests for HTTP status codes.

Summary:
PR description:

This adds explicit tests for the returned HTTP status codes to
interface_rpc.py (for error cases) and the HTTP JSON-RPC client in
general for success.

PR15381 brought up discussion about the HTTP status codes in general,
and the general opinion was that the current choice may not be ideal
but should not be changed to preserve compatibility with existing
JSON-RPC clients. Thus it makes sense to actually test the current
status to ensure this desired compatibility is not broken accidentally.

This is a backport of Core PR15495

Test Plan: ninja && test/functional/test_runner.py interface_rpc

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7752