Move traits from core/lib/Drupal/Core/Test to core/tests

Added some formatting to the IS. Opened an MR to get the ball rolling.

1. Moved Drupal\Core\Test\FunctionalTestSetupTrait to \Drupal\Tests\FunctionalTestSetupTrait. Arguably it could go in Drupal\Tests\Traits but it's not clear when to use that and when not to. It could also go in Drupal\FunctionalTests but it's also used by \Drupal\TestSite\Commands\TestSiteInstallCommand so I don't know if a more generic namespace is better.
2. Moved Drupal\Core\Test\RefreshVariablesTrait to Drupal\Tests\RefreshVariablesTrait. Again, it could go in Drupal\Tests\Traits but it seems logical to put it next to UiHelperTrait.

I think it's just these three that can move.

• \Drupal\Core\Test\AssertMailTrait
• \Drupal\Core\Test\FunctionalTestSetupTrait
• \Drupal\Core\Test\RefreshVariablesTrait

Probably \Drupal\Core\Test\TestSetupTrait too

Directly in Drupal\Tests seems OK, we already have lots of traits in there as you point out.

I wonder how many contrib tests will break because there aren't deprecated sub-traits remaining... Also DTT might have to do a re-organize.

Arguably it could go in Drupal\Tests\Traits but it's not clear when to use that and when not to.

The Traits namespace is there so we can limit the number of autoloaded test files per framework. Since some frameworks share traits, we put them in a separate directory so we can autoload them without autoloading everything else. This only applies to modules, AFAICR. https://git.drupalcode.org/project/drupal/-/blob/11.x/core/tests/bootstr... See also TestSuiteBase and its subclasses. https://git.drupalcode.org/project/drupal/-/blob/11.x/core/tests/TestSui...

It might help us to do the same thing for non-module core, but I doubt it.

Added a deprecated version of AssertMailTrait, if that looks right I'll add a CR and deprecate the others too. Do we need deprecation tests then too?

Believe we need a CR for the deprecations vs pointing to the ticket

Seeking feedback on #9 but will probably just go ahead with the others.