Remove usage of DOING_AJAX in the test suite

[johnbillion]

In the test suite, several tests define DOING_AJAX to true. This effectively pollutes the tests which follow them, and prevent any other tests from operating in a non-Ajax state when they need to (for example, Tests_WP_Customize_Manager::test_not_doing_ajax()).

Instances of DOING_AJAX should be replaced by use of a filter, for example add_filter( 'wp_doing_ajax', '__return_true' ).

[johnbillion]

Previously: #25669

[Mte90]

I was looking on simple tickets to start with unit tests so this was perfect.
I checked and with this changes there was no problems with the tests :-)

[johnbillion]

@Mte90 Looks good!

1. The test_not_doing_ajax() test can use add_filter( 'wp_doing_ajax', '__return_false' ) to force a non-Ajax request, and then remove the markTestSkipped call.
2. Does this cover all instances of DOING_AJAX in the test suite?

[Mte90]

1. I will do a new patch for that asap
2. I searched in all the code about the use of the constant and replace with the function :-)

[Mte90]

removed marktestskipped

[johnbillion]

In 41293:

Build/Test tools: Remove usage of DOING_AJAX from the test suite, so all tests that expect either an Ajax request or a
non-Ajax request can operate without being skipped.

Props Mte90

Fixes #41561

[matthias.thiel]

I am not sure the current implementation resolves the issue properly. Adding a filter in the static setUpBeforeClass method will still exist in test cases that are run later. I advise putting the hook into the setUp method like the wp_die_ajax_handler hook.

I think this would also be more in line with the WP_UnitTestCase class as it backs up all hooks in the setUp method.

[johnbillion]

Re-opening as per comment above.

[peterwilsoncc]

41561.3.diff​ moves the filter from setUpBeforeClass() to setUp() in WP_Ajax_UnitTestCase. All other occurrences are in the correct location.

[peterwilsoncc]

In 41970:

Build/Test tools: Move wp_doing_ajax defintion from class setup to test setup.

Moves defintion of ajax request in WP_Ajax_UnitTestCase to setUp() method to account for hooks being reset as part of tearDown().

Props matthias.thiel for report.
Fixes #41561.