Fix broken the post-template-editor test. (#32904)

* Change to a theme that supports block-templates, and then delete wp_template and wp_template_part posts.

* add plugin for switch add_theme_support('block-templates')

* Update packages/e2e-tests/specs/experiments/post-editor-template-mode.test.js

Co-authored-by: Greg Ziółkowski <grzegorz@gziolo.pl>

* Update packages/e2e-tests/specs/experiments/post-editor-template-mode.test.js

Co-authored-by: Greg Ziółkowski <grzegorz@gziolo.pl>

packages/e2e-tests/plugins/block-templates.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * Plugin Name: Gutenberg Test Block Templates
+ * Plugin URI: https://github.com/WordPress/gutenberg
+ * Author: Gutenberg Team
+ *
+ * @package gutenberg-test-block-templates
+ */
+
+/**
+ * Activate Block Templates.
+ */
+function enqueue_block_templates() {
+	add_theme_support( 'block-templates' );
+}
+
+add_action( 'setup_theme', 'enqueue_block_templates' );

packages/e2e-tests/specs/experiments/post-editor-template-mode.test.js

@@ -9,6 +9,8 @@ import {
 	trashAllPosts,
 	openPreviewPage,
 	openDocumentSettingsSidebar,
+	activatePlugin,
+	deactivatePlugin,
 } from '@wordpress/e2e-test-utils';
 
 const openSidebarPanelWithTitle = async ( title ) => {
@@ -89,12 +91,14 @@ const createNewTemplate = async ( templateName ) => {
 
 describe( 'Post Editor Template mode', () => {
 	beforeAll( async () => {
+		await activatePlugin( 'gutenberg-test-block-templates' );
 		await trashAllPosts( 'wp_template' );
 		await trashAllPosts( 'wp_template_part' );
 	} );
 
 	afterAll( async () => {
 		await activateTheme( 'twentytwentyone' );
+		await deactivatePlugin( 'gutenberg-test-block-templates' );
 	} );
 
 	it( 'Allow to switch to template mode, edit the template and check the result', async () => {