[FEATURE] Add FakeMailer inspired by Laravel

[simonschaufi]

See https://laravel.com/docs/10.x/mail#testing-mailable-sending for inspiration source and https://github.com/laravel/framework/blob/10.x/src/Illuminate/Support/Testing/Fakes/MailFake.php for the original source code.

How to use:

$mailer = GeneralUtility::makeInstance(FakeMailer::class);
GeneralUtility::setSingletonInstance(Mailer::class, $mailer);

// run code which should send an email (dispatch controller)

$mailer->assertSent(FluidEmail::class, 1);
$mailer->assertSent(FluidEmail::class, function (FluidEmail $mail) {
    $textBody = $mail->getTextBody();

    return $mail->getSubject() === 'Subject'
        && str_contains($textBody, 'First name: Anna')
        && in_array(new Address('[email protected]', 'My Company'), $mail->getFrom(), false);
});

[lolli42]

Hey.

We decided to not implement this, for similar reasons as with #524

Additionally, this creates quite a dependency to core class Mailer, which would have to be maintained in the future.

I hope it is ok to close here for now.