Description
Since 4.4, we have new PHPUnit constraints for the mailer component, but these are only available with WebTestCase and not KernelTestCase. I think this would be useful to access these constraints from both TestCase. If the email is queued, we could use the assertQueuedEmailCount() function.

Example
For instance, I have a service which sends an email with an attached document, I would like to test that the email is correctly sent and that there is an attached document with it.