Tests: Increase test coverage for WP_Theme_JSON_Resolver.

This commit adds a unit test to check that `WP_Theme_JSON_Resolver::get_theme_data()` returns a `WP_Theme_JSON` object, and that an `add_theme_support()` call overrides the settings from `theme.json`.

Follow-up to [54443], [54493].

Props cbravobernal.
Fixes #56835.
Built from https://develop.svn.wordpress.org/trunk@54630


git-svn-id: https://core.svn.wordpress.org/trunk@54182 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-RC1-54629';
+$wp_version = '6.1-RC1-54630';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.