Finish the plugin system integration tests

Adds the last three checks from #342 to test_plugin: an InstanceEcho
request handled on a running instance, a Lua module with a remote
interface that the request calls over RCON, and log tagging checks for
the plugin field and host/instance ids on the controller, host and
instance log lines.

test_plugin now has a module, so the create_scenario and patch unit
tests that relied on it having none use global_chat instead.

Closes #342

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/file/test_plugin/instance.js

@@ -1,3 +1,7 @@
+import * as lib from "@clusterio/lib";
+
+import { InstanceEcho } from "./messages.js";
+
 export default async function(context) {
 	const { instance, plugin, logger } = context;
 	logger.info("test_plugin instance loaded");
@@ -5,4 +9,11 @@ export default async function(context) {
 	instance.hooks.start.attach(plugin.name, async () => {
 		logger.info("test_plugin instance started");
 	});
+
+	instance.handle(InstanceEcho, async (request) => {
+		logger.info(`test_plugin instance echo ${request.text}`);
+		const text = lib.escapeString(request.text);
+		const response = await instance.sendRcon(`/sc rcon.print(remote.call("test_plugin", "echo", "${text}"))`);
+		return response.replace(/\n$/, "");
+	});
 }