Fix FunctionalJavascript test classes must specify the #[RunTestsInSeparateProcesses] attribute

Problem/Motivation

phpunit jobs at 'next minor' running 11.3-dev have started to show this deprecation, which may be the cause of the jobn ending with an amber warning not a green pass.

1) /builds/project/gitlab_templates_downstream/web/core/tests/Drupal/Tests/BrowserTestBase.php:334
Functional/FunctionalJavascript test classes must specify the #[RunTestsInSeparateProcesses] attribute, not doing so is deprecated in drupal:11.3.0 and is throwing an exception in drupal:12.0.0. See https://www.drupal.org/node/3548485

Triggered by:

* Drupal\Tests\gitlab_templates_downstream\Functional\GitlabTemplatesDownstreamTwoTest::testDownstream
/builds/project/gitlab_templates_downstream/tests/src/Functional/GitlabTemplatesDownstreamTwoTest.php:22

Steps to reproduce

See recent pipelines, eg https://git.drupalcode.org/project/gitlab_templates_downstream/-/jobs/6903357

Proposed resolution

Add the attribute