Convert tests to PHPUnit

1. +++ b/tests/src/Functional/AdminFormSettingsTest.php
   @@ -1,29 +1,26 @@
       */
   -  public static $modules = [
   -    'mailsystem'
   -  ];
   +  public static $modules = ['mailsystem'];
   

   core is in the process of changing this to protected, you should be able to do that already.

2. +++ b/tests/src/Functional/MailsystemTestThemeTest.php
   @@ -1,15 +1,19 @@
   +class MailsystemTestThemeTest extends BrowserTestBase {
   +  use AssertMailTrait {
   +    getMails as drupalGetMails;
   +  }
    
   

   the as drupalX use statements are for BC, to use the old method names, if you include this yourself, I'd just update the method calls and just do use AssertMailTrait;

Those are just nitpicks, looks fine I think.

I do not know if I understood correctly, would that be?

I think so, the interdiff is somehow broken, so a bit hard to review, but the use and public/protected is changed.