Tests: Correct newly introduced @covers tags.

When global functions are covered, they need to be prefixed with `::` (double colon) to distinguish them from class name.

See https://phpunit.readthedocs.io/en/7.0/annotations.html#covers for more details.

Follow-up to [49000], [49171].

Props jrf.
See #39265.

git-svn-id: https://develop.svn.wordpress.org/trunk@49305 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/taxonomy.php

@@ -1049,7 +1049,7 @@ class Tests_Taxonomy extends WP_UnitTestCase {
 	/**
 	 * Ensure custom callbacks are used when registered.
 	 *
-	 * @covers register_taxonomy
+	 * @covers ::register_taxonomy
 	 * @ticket 40351
 	 */
 	function test_register_taxonomy_counting_callbacks() {