$defaultTheme being set via a Trait is not discovered.

[dpagini]

Bug report

First, let me say that I think this is an edge case, so I really don't know if there's value in trying to solve this... would be totally OK if this gets closed as "wontfix".

In my particular issue, when Drupal announced that $defaultTheme must be set for all tests, in my project we started setting this via a trait, but the trait sets the property at runtime. Given this is a very unique use case, I would be happy to just ignore this rule, so I opened #772 which would help me ignore it.

I'm not totally sure if this is a useful bug report for other reasons.. just to consider that the property may be set outside the test itself...?

Code snippet that reproduces the problem

Here is how I set $defaultTheme via trait...

trait ThemeTestTrait {

  /**
   * {@inheritdoc}
   *
   * Set all custom tests to use stark by default.
   *
   * This was introduced in Drupal core v9 where all tests switched from the
   * classy theme to an undefined theme.
   *
   * @see https://www.drupal.org/node/3083055
   */
  protected function installDefaultThemeFromClassProperty(ContainerInterface $container): void {
    if (!isset($this->defaultTheme)) {
      $this->defaultTheme = 'stark';
    }

    parent::installDefaultThemeFromClassProperty($container);