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.
PR 15381 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 adds a new regtest file 'interface_rpc.py', containing a test for
batch JSON-RPC requests. Those were previously not tested at all. Tests
for basic requests are not really necessary, as those are used anyway
in lots of other regtests.
The existing interface_http.py file is more about the underlying HTTP
connection, so adding a new interface file for the JSON-RPC specific
things makes sense.
