Coding Standards: Add visibility to properties in tests/phpunit/tests/.

Adds a `private` visibility to some test class properties where it was not explicitly specified. This continues the previous efforts to make sure visibility is declared on all properties. 

Note: This will be enforced by WPCS 3.0.0.

Follow-up to [49184], [51919], [52009], [52010], [54889].

Props jrf.
See #56791.

git-svn-id: https://develop.svn.wordpress.org/trunk@54890 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/query/fieldsClause.php

@@ -11,14 +11,14 @@ class Tests_Query_FieldsClause extends WP_UnitTestCase {
 	 *
 	 * @var int[]
 	 */
-	static $post_ids = array();
+	private static $post_ids = array();
 
 	/**
 	 * Page IDs.
 	 *
 	 * @var int[]
 	 */
-	static $page_ids = array();
+	private static $page_ids = array();
 
 	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
 		// Register CPT for use with shared fixtures.