Themes: Add block template resolution algorithm unit tests.

Backports the unit tests for block templates
and hybrid themes template resolution from the Gutenberg plugin.

Props bernhard-reiter, jorgefilipecosta.
Fixes #54478. 


git-svn-id: https://develop.svn.wordpress.org/trunk@52246 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/block-template-utils.php

@@ -7,6 +7,8 @@
 
 /**
  * Tests for the Block Templates abstraction layer.
+ *
+ * @group block-templates
  */
 class Block_Template_Utils_Test extends WP_UnitTestCase {
 	private static $post;