[7.x] Allow doing truth-test assertions with just a closure

[SjorsO]

Currently, for all the truth-test assertions, you have to first pass in a class, then a closure. If you want your editor to assist you with auto-completion, you have to type the class twice. Once as the first argument for the assertQueued method, and once as the type hint for the parameter of the closure.

Mail::assertQueued(SubpictureFinishedEmail::class, function (SubpictureFinishedEmail $email) use ($user) {
    return $email->user->id === $user->id;
});

// or with a short closure

Mail::assertQueued(SubpictureFinishedEmail::class, fn (SubpictureFinishedEmail $email) => $email->user->id === $user->id);

This PR, using a little reflection magic, infers the type from the parameter of the closure. This removes the duplication, and allows you to write the assertions like this instead:

Mail::assertQueued(function (SubpictureFinishedEmail $email) use ($user) {
    return $email->user->id === $user->id;
});

// or with a short closure

Mail::assertQueued(fn (SubpictureFinishedEmail $email) => $email->user->id === $user->id);

This isn't a breaking change, all modified methods will currently throw an exception if you give them a closure as the first argument.

[taylorotwell]

Nice PR. Thanks ❤️