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/host/patch.js

@@ -241,7 +241,7 @@ describe("host/patch", function() {
 		});
 		it("should skip plugins without a module", async function() {
 			const pluginInfos = await lib.loadPluginInfos(
-				new Map([["test_plugin", path.resolve("test/file/test_plugin")]])
+				new Map([["global_chat", path.resolve("plugins/global_chat")]])
 			);
 			const modules = await patch.loadModules(pluginInfos);
 			assert.deepEqual([...modules.keys()], ["clusterio"]);