Build/Test Tools: Expand unit tests for `theme.json`.

This changeset adds unit test coverage for the following features:

- Gradients
- `filter` (Duotones)
- `blockGap`
- `shadow`
- `useRootPaddingAwareAlignments`
- `appearanceTools`

Props gunterer, johnregan3, audrasjb, mukesh27.
Fixes #56611.

Built from https://develop.svn.wordpress.org/trunk@54443


git-svn-id: http://core.svn.wordpress.org/trunk@54002 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-beta3-54442';
+$wp_version = '6.1-beta3-54443';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.