Add regtest for JSON-RPC batch calls.

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.

test/functional/test_runner.py

@@ -120,6 +120,7 @@ BASE_SCRIPTS = [
     'wallet_disableprivatekeys.py',
     'wallet_disableprivatekeys.py --usecli',
     'interface_http.py',
+    'interface_rpc.py',
     'rpc_psbt.py',
     'rpc_users.py',
     'feature_proxy.py',