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/external-http/basic.php

@@ -9,7 +9,7 @@ class Tests_External_HTTP_Basic extends WP_UnitTestCase {
 	 * @covers ::wp_remote_retrieve_response_code
 	 * @covers ::wp_remote_retrieve_body
 	 */
-	function test_readme() {
+	public function test_readme() {
 		// This test is designed to only run on trunk/master.
 		$this->skipOnAutomatedBranches();