add mu plugin to lower the autosave interval for testing

test/e2e/test-mu-plugins/low-autosave-interval.php

@@ -0,0 +1,11 @@
+<?php
+/**
+ * Plugin Name: Low Autosave Interval
+ * Plugin URI: https://github.com/WordPress/gutenberg
+ * Author: Gutenberg Team
+ *
+ * @package gutenberg-test-low-autosave-interval
+ */
+if ( ! defined( 'AUTOSAVE_INTERVAL' ) ) {
+	define( 'AUTOSAVE_INTERVAL', 3 );
+}