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/module/test_plugin.lua

@@ -0,0 +1,5 @@
+remote.add_interface("test_plugin", {
+	echo = function(text)
+		return text
+	end,
+})