[BUGFIX] Adapt SendEmailCommandTest for symfony/mailer

The tests produced a warning which didn't cause CI to abort,
but the test was actually skipped:

1) TYPO3\CMS\Core\Tests\Unit\Command\SendEmailCommandTest
     ::executeWillFlushTheQueue
   Trying to configure method "flushQueue" which cannot be configured
   because it does not exist, has not been specified, is final, or is
   static

Resolves: #88692
Related: #88643
Releases: master
Change-Id: I3077a011e19050986585e9c788f5db03d18595d3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61233


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Alexander Schnitzler <review.typo3.org@alexanderschnitzler.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Alexander Schnitzler <review.typo3.org@alexanderschnitzler.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>