pytest: demonstrate failing rpc_command hook rewrite to plugin.

lightningd: lightningd/plugin.c:648: plugin_rpcmethod_dispatch: Assertion `idtok != NULL' failed.
lightningd: FATAL SIGNAL 6 (version v0.7.3-234-gf3aeb51-modded)
0x557652363eb1 send_backtrace
	common/daemon.c:41
0x557652363f5b crashdump
	common/daemon.c:54
0x7f5b065a346f ???
	???:0
0x7f5b065a33eb ???
	???:0
0x7f5b06582898 ???
	???:0
0x7f5b06582768 ???
	???:0
0x7f5b06594005 ???
	???:0
0x557652355901 plugin_rpcmethod_dispatch
	lightningd/plugin.c:648
0x557652331836 command_exec
	lightningd/jsonrpc.c:588
0x557652331bad rpc_command_hook_callback
	lightningd/jsonrpc.c:655
0x557652358745 plugin_hook_callback
	lightningd/plugin_hook.c:90
0x55765235486a plugin_response_handle
	lightningd/plugin.c:258
0x557652354a16 plugin_read_json_one
	lightningd/plugin.c:356
0x557652354b6d plugin_read_json
	lightningd/plugin.c:388
0x5576523b5e6c next_plan
	ccan/ccan/io/io.c:59
0x5576523b6a51 do_plan
	ccan/ccan/io/io.c:407
0x5576523b6a93 io_ready
	ccan/ccan/io/io.c:417
0x5576523b8d2b io_loop
	ccan/ccan/io/poll.c:445
0x55765232dc6f io_loop_with_timers
	lightningd/io_loop_with_timers.c:24
0x557652334a6e main
	lightningd/lightningd.c:848
0x7f5b065841e2 ???
	???:0
0x5576523193ed ???
	???:0
0xffffffffffffffff ???
	???:0
Log dumped in crash.log.20191206093336

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/rpc_command.py

@@ -21,6 +21,9 @@ def on_rpc_command(plugin, rpc_command, **kwargs):
         # Don't allow this command to be executed
         return {"return": {"error": {"code": -1,
                                      "message": "You cannot do this"}}}
+    elif request["method"] == "help":
+        request["method"] = "autocleaninvoice"
+        return {"replace": request}
     return {"continue": True}