[8.x] Allow for closure reflection on all MailFake assertions

[inxilpro]

Right now, only the positive assertions on the MailFake class allow for passing a closure rather than a FQCN and a closure:

// This works:
Mail::assertSent(fn(MyMailable $mail) => $mail->hasTo('foo@bar.com'));

// This doesn't:
Mail::assertNotSent(fn(MyMailable $mail) => $mail->hasTo('foo@bar.com'));

// Instead, right now you must do:
Mail::assertNotSent(MyMailable::class, fn(MyMailable $mail) => $mail->hasTo('foo@bar.com'));

This PR updates all the MailFake assertions to allow for the single closure API.