Add result shorthands for Process fakes

[jasonmccreary]

This adds two new shorthands for faking process results in tests. The first is a shorthand to simply set the exit code. The second is a shorthand to throw an exception.

Process::fake([
    'php -l script.php' => 255, 
    'cat README.txt' => new \RuntimeException('fake exception message'),
]);

Note: while the latter is not common as it is only possible for the underlying Symfony Process to throw a RuntimeException. This path was hard to test unless a fake process can throw an exception - basically you had to single out the command in a callback, then throw. This is cleaner and can be used to test other paths, such as process timeout, etc.

[taylorotwell]

Thanks!