Convert simpletest to PHPUnit test

Looks fine to me, just one nitpick, lets include the $defaultTheme = 'stark' definition in the new test so we don't have to update it again?

Added $defaultTheme = 'stark' in test. Please review.

1. +++ b/tests/src/FunctionalJavascript/SwiftMailerSettingsTest.php
   @@ -0,0 +1,140 @@
   +  /**
   +   * {@inheritdoc}
   +   */
   +  protected $defaultTheme = 'stark';
   +  ¶
   +  /**
   +   * Modules to enable.
   

   extra spaces.

2. +++ b/tests/src/FunctionalJavascript/SwiftMailerSettingsTest.php
   @@ -0,0 +1,140 @@
   +
   +  protected $adminUser;
   

   lets add a docblock for this while moving it around. Admin user as description and @var \Drupal\user\UserInterface.

Make sure you select a compatible PHP version when adding a test.

Made suggested changes. Please review.

+++ b/tests/src/FunctionalJavascript/SwiftMailerSettingsTest.php
@@ -0,0 +1,145 @@
+  /**
+   * The user object.
+   *
+   * @var \Drupal\user\UserInterface.
+   */
+  protected $adminUser;

no . at the end of @var, but that can be fixed on commit.

More importantly, when making changes to an existing patch, always provide an interdiff, so it is easier to review.

Reroll, without the test theme change.

Ok, so this now actually fails because I forgot to add the key to the mailsystem test theme, will need to fix that.

This passes on 8.8, so I think you could also commit it.

Agree, committed.

I reopened #3096960: Drupal 9 deprecated code report for the remaining error in mailsystem_test_theme.