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:
Follow-up to [52298], [52145], [52052].
Props antonvlasenko, costdev, ironprogrammer, robinwpdeveloper, hellofromTonya.
Fixes #56266.