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

This commit adds the `public` visibility keyword to each method which did not have an explicit visibility keyword.

Why `public`?

With no visibility previously declared, these methods are implicitly `public` and available for use. As these are part of the WordPress testing framework (for Core and extenders), changing them to anything else would be a backwards-compatibility break.

Props costdev, jrf, hellofromTonya.
See #54177.

git-svn-id: https://develop.svn.wordpress.org/trunk@52009 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/mock-mailer.php

@@ -5,7 +5,7 @@ require_once ABSPATH . 'wp-includes/PHPMailer/Exception.php';
 class MockPHPMailer extends PHPMailer\PHPMailer\PHPMailer {
 	public $mock_sent = array();
 
-	function preSend() {
+	public function preSend() {
 		$this->Encoding = '8bit';
 		return parent::preSend();
 	}
@@ -13,7 +13,7 @@ class MockPHPMailer extends PHPMailer\PHPMailer\PHPMailer {
 	/**
 	 * Override postSend() so mail isn't actually sent.
 	 */
-	function postSend() {
+	public function postSend() {
 		$this->mock_sent[] = array(
 			'to'      => $this->to,
 			'cc'      => $this->cc,