pytest: Add a test for the plugin option passthrough

Signed-off-by: Christian Decker <@cdecker>

contrib/plugins/helloworld.py

@@ -0,0 +1,70 @@
+#!/usr/bin/env python3
+"""Simple plugin to show how to build new plugins for c-lightning
+
+It demonstrates how a plugin communicates with c-lightning, how it registers
+command line arguments that should be passed through and how it can register
+JSON-RPC commands. We communicate with the main daemon through STDIN and STDOUT,
+reading and writing JSON-RPC requests.
+
+"""
+import json
+import sys
+
+
+greeting = "World"
+
+
+def json_hello(request):
+    greeting = "Hello {}".format(request['params']['name'])
+    return greeting
+
+
+def json_getmanifest(request):
+    global greeting
+    return {
+        "options": [
+            {"name": "greeting",
+             "type": "string",
+             "default": greeting,
+             "description": "What name should I call you?"},
+        ],
+        "rpcmethods": [
+            {
+                "name": "hello",
+                "description": "Returns a personalized greeting for {name}",
+            },
+        ]
+    }
+
+
+def json_init(request):
+    """The main daemon is telling us the relevant cli options
+    """
+    global greeting
+
+    greeting = request['params']['options']['greeting']
+    return "ok"
+
+
+methods = {
+    'hello': json_hello,
+    'getmanifest': json_getmanifest,
+    'init': json_init,
+}
+
+partial = ""
+for l in sys.stdin:
+    partial += l
+    try:
+        request = json.loads(partial)
+        result = {
+            "jsonrpc": "2.0",
+            "result": methods[request['method']](request),
+            "id": request['id']
+        }
+        json.dump(result, fp=sys.stdout)
+        sys.stdout.write('\n')
+        sys.stdout.flush()
+        partial = ""
+    except Exception:
+        pass