[5.5] Assert how many times a mail was sent or a job was pushed to the queue

[mateusjatenee]

Hey guys,

This is not a huge or important PR, but I think it would be a nice addition. It allows people to assert how many times a job was pushed to the queue or a mail was sent.

The method names are pretty bad but I couldn't think of anything else and didn't wanna mess with the default implementation.. perhaps check if $mailable is an array on the normal methods and call this different ones? like Mail::assertSent([Foo:class, 3])

Edit:

I added a commit that allows you to do this:

Mail::assertSent([Foo::class, 2]);

Instead of this Mail::assertSentTimes(Foo::class, 2);

[taylorotwell]

Cleaned up some syntax and variable errors. Also callbacks aren't needed at all in this scenario.