Tests: Enable post type UI for templates and template parts (#38486)

* Tests: Enable post type UI for templates and template parts
* Only enable UI for block themes

packages/e2e-tests/mu-plugins/enable-templates-ui.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * Plugin Name: Gutenberg Test Plugin, Enable Templates UI
+ * Plugin URI: https://github.com/WordPress/gutenberg
+ * Author: Gutenberg Team
+ *
+ * @package gutenberg-test-enable-templates-ui
+ */
+
+/**
+ * Enable Templates & Template Parts post type UI during e2e testing.
+ */
+add_filter(
+	'register_post_type_args',
+	static function( $args, $name ) {
+		if ( in_array( $name, array( 'wp_template', 'wp_template_part' ), true ) ) {
+			$args['show_ui'] = wp_is_block_theme();
+		}
+
+		return $args;
+	},
+	20,
+	2
+);