Build/Test Tools: Introduce skipWithoutMultisite() and skipWithMultisite() methods into the test suite.

This brings much needed uniformity to test skipping when a test requires Multisite or when a test should be excluded from running when Multisite is enabled.

Used in conjunction with the `@group ms-required` and `@group ms-excluded` notation, this removes a significant number of skipped tests from the default test suite run.

Fixes #40531


git-svn-id: https://develop.svn.wordpress.org/trunk@40543 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user.php

@@ -652,9 +652,7 @@ class Tests_User extends WP_UnitTestCase {
 	 * @group ms-required
 	 */
 	function test_illegal_user_logins_multisite() {
-		if ( ! is_multisite() ) {
-			$this->markTestSkipped( __METHOD__ . ' requires multisite.' );
-		}
+		$this->skipWithoutMultisite();
 
 		$user_data = array(
 			'user_login' => 'testuser',