TestCase: stabilize handling of static properties in PHPMailer class

Public static properties changed by individual tests were not being reset to their default value prior to the next test being run.
This could influence the test results of subsequent tests as static properties are not automatically reset when a new instance of a class is instantiated. See: https://3v4l.org/8s1RB

Luckily this didn't aversely affect the tests so far, but should be safeguarded for the future.

There are only three static properties in the `PHPMailer` class, with only one of these being `public`.

This commit introduces a code snippet which will reset any static properties as listed in the `$PHPMailerStaticProps` property of the `TestCase` to the default value, as also set in the `$PHPMailerStaticProps` property, at the start of the `set_up()` which is run before each test using the `TestCase`.

For now, this is only relevant for the `ValidateAddressCustomValidatorTest` test and only when the `testSetDefaultValidatorToCustom()` test would fail before resetting the property. All the same, having the reset in place by default will ensure that future tests which change this property won't introduce side-effects to other tests.

test/TestCase.php

@@ -50,6 +50,17 @@ abstract class TestCase extends PolyfillTestCase
      */
     private $NoteLog = [];
 
+    /**
+     * List of *public static* properties in the PHPMailer class with their default values.
+     *
+     * This list is used by the `set_up()` method.
+     *
+     * @var array Key is the property name, value the default as per the PHPMailer class.
+     */
+    private $PHPMailerStaticProps = [
+        'validator' => 'php',
+    ];
+
     /**
      * Run before each test class.
      */
@@ -69,6 +80,11 @@ abstract class TestCase extends PolyfillTestCase
      */
     protected function set_up()
     {
+        // Make sure that public static variables contain their default values at the start of each test.
+        foreach ($this->PHPMailerStaticProps as $key => $value) {
+            PHPMailer::${$key} = $value;
+        }
+
         if (file_exists(\PHPMAILER_INCLUDE_DIR . '/test/testbootstrap.php')) {
             include \PHPMAILER_INCLUDE_DIR . '/test/testbootstrap.php'; //Overrides go in here
         }