Remove FormattableMarkup from FileManagedUnitTestBase

Left some comments/questions on MR.

I think we need to change issue title because method assertFileHookCalled() is only place at kernel tests still used FormattableMarkup. And assertFileHookCalled() is also presented at Drupal\Tests\file\Functional\FileManagedTestBase.
Should we move this method to trait?

Now it's ready for review.

Believe feedback has been addressed.

Committed and push to 11.x

Thanks!

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