Add regtests for HTTP status codes.

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.

test/functional/interface_rpc.py

@@ -1,12 +1,22 @@
 #!/usr/bin/env python3
-# Copyright (c) 2018 The Bitcoin Core developers
+# Copyright (c) 2018-2019 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Tests some generic aspects of the RPC interface."""
 
+from test_framework.authproxy import JSONRPCException
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_greater_than_or_equal
 
+def expect_http_status(expected_http_status, expected_rpc_code,
+                       fcn, *args):
+    try:
+        fcn(*args)
+        raise AssertionError("Expected RPC error %d, got none" % expected_rpc_code)
+    except JSONRPCException as exc:
+        assert_equal(exc.error["code"], expected_rpc_code)
+        assert_equal(exc.http_status, expected_http_status)
+
 class RPCInterfaceTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
@@ -48,9 +58,16 @@ class RPCInterfaceTest(BitcoinTestFramework):
         assert_equal(result_by_id[3]['error'], None)
         assert result_by_id[3]['result'] is not None
 
+    def test_http_status_codes(self):
+        self.log.info("Testing HTTP status codes for JSON-RPC requests...")
+
+        expect_http_status(404, -32601, self.nodes[0].invalidmethod)
+        expect_http_status(500, -8, self.nodes[0].getblockhash, 42)
+
     def run_test(self):
         self.test_getrpcinfo()
         self.test_batch_request()
+        self.test_http_status_codes()
 
 
 if __name__ == '__main__':