Add declare(strict_types=1) to all test modules

With the (temporary) modification of phpstan.neon, I can see there is an unused test method (doFlagUiFieldPlugin()) that uses an undefined core method (drupalPostAjaxForm()). I opened up 🐛 drupalPostAjaxForm() does not exist anymore Active for that issue.

Opened up a new issue for this:
Installing the table sequences with the method KernelTestBase::installSchema() is deprecated in drupal:10.2.0 and is removed from drupal:12.0.0.
See 📌 [10.2] Drupal\Core\Database\Connection::nextId(), the {sequences} table and schema are deprecated Active

All phpstan issue related to turning on strict_types in the tests have now been resolved, with the exception of 🐛 drupalPostAjaxForm() does not exist anymore Active , which requires changes that IMO are outside the scope of this issue.

Good job, TR!

Automatically closed - issue fixed for 2 weeks with no activity.