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

Adds a `public` visibility to test fixtures, tests, data providers, and callbacks methods.

Adds a `private` visibility to helper methods within test classes.

Renames callbacks and helpers that previously started with a `_` prefix. Why? For consistency and to leverage using the method visibility. Further naming standardizations is beyond the scope of this commit.

Props costdev, jrf, hellofromTonya.
Fixes #54177.

git-svn-id: https://develop.svn.wordpress.org/trunk@52010 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/date/getFeedBuildDate.php

@@ -8,7 +8,7 @@
  */
 class Tests_Date_GetFeedBuildDate extends WP_UnitTestCase {
 
-	function tear_down() {
+	public function tear_down() {
 		global $wp_query;
 
 		update_option( 'timezone_string', 'UTC' );