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/.eslintrc.js

@@ -0,0 +1,9 @@
+module.exports = {
+  settings: {
+    "import/resolver": {
+      typescript: {
+        project: __dirname,
+      },
+    },
+  },
+}