Editor: Add PHPUnit tests for 5.9.0 new functions.

During the 5.9.0 cycle, tests were missed during the porting from Gutenberg to Core for the following functions:

* `_disable_block_editor_for_navigation_post_type()`
* `_disable_content_editor_for_navigation_post_type()`
* `_enable_content_editor_for_navigation_post_type()`
* `wp_filter_global_styles_post()`

This commit adds new test classes for these functions.

Reference:
* [https://github.com/WordPress/gutenberg/blob/release/13.6/phpunit/global-styles-test.php Gutenberg v13.6] for `WP_Global_Styles_Test`
* [https://github.com/WordPress/gutenberg/blob/release/13.6/phpunit/navigation-test.php Gutenberg v13.6] for `WP_Navigation_Test`

Follow-up to [52298], [52145], [52052].

Props antonvlasenko, costdev, ironprogrammer, robinwpdeveloper, hellofromTonya.
Fixes #56266.
Built from https://develop.svn.wordpress.org/trunk@54382


git-svn-id: http://core.svn.wordpress.org/trunk@53941 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-beta2-54381';
+$wp_version = '6.1-beta2-54382';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.