Tests: Clean up skipping conditions and requirements for various tests.

This improves the consistency of test skipping and ensures that:

* The `@requires` annotations use the right condition and format, and are on the right level (class vs. function).
* Inline conditions with a `markTestSkipped()` call are only used when annotations cannot be used.
* All `markTestSkipped()` calls contain a verbose explanation of why the test is being skipped.

Props jrf, hellofromTonya.
Fixes #53009.

git-svn-id: https://develop.svn.wordpress.org/trunk@51415 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/image/editorGd.php

@@ -53,6 +53,8 @@ class Tests_Image_Editor_GD extends WP_Image_UnitTestCase {
 
 	/**
 	 * Test resizing an image, not using crop
+	 *
+	 * @requires function imagejpeg
 	 */
 	public function test_resize() {
 		$file = DIR_TESTDATA . '/images/waffles.jpg';
@@ -73,6 +75,8 @@ class Tests_Image_Editor_GD extends WP_Image_UnitTestCase {
 
 	/**
 	 * Test multi_resize with single image resize and no crop
+	 *
+	 * @requires function imagejpeg
 	 */
 	public function test_single_multi_resize() {
 		$file = DIR_TESTDATA . '/images/waffles.jpg';
@@ -183,6 +187,7 @@ class Tests_Image_Editor_GD extends WP_Image_UnitTestCase {
 	 * Test multi_resize with multiple sizes
 	 *
 	 * @ticket 26823
+	 * @requires function imagejpeg
 	 */
 	public function test_multi_resize() {
 		$file = DIR_TESTDATA . '/images/waffles.jpg';