refactor: move test-plugin to integration suite

This seems more appropriate given this tests how a plugin might work
within code-server.

test/integration/test-plugin/package.json

@@ -0,0 +1,16 @@
+{
+  "private": true,
+  "name": "test-plugin",
+  "version": "1.0.0",
+  "engines": {
+    "code-server": "^4.7.0"
+  },
+  "main": "out/index.js",
+  "devDependencies": {
+    "@types/express": "^4.17.8",
+    "typescript": "^4.0.5"
+  },
+  "scripts": {
+    "build": "tsc"
+  }
+}