Tests: Cover Block Hooks integration with a custom block theme

Adds a simplified version of Twenty Twenty-Three theme that helps testing Block Hooks integration. The theme contains:

- The required index.html template.
- The optional single.html template used with tests.
- 3 template parts where two of them reference patterns.
- 3 patterns referenced in the templates and the template parts.

New tests automatically register 4 custom blocks with the test theme where each of them hooks into another block using all four target relative positions: `before`, `after`, `firstChild`, `lastChild`.

The tests verify that the block gets hooked into the correct positions when targeting:

- template
- template part
- pattern

Props ockham, costdev.
See #59313, #59383.
Follow-up [56610].

Built from https://develop.svn.wordpress.org/trunk@56759


git-svn-id: http://core.svn.wordpress.org/trunk@56271 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-beta1-56757';
+$wp_version = '6.4-beta1-56759';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.