[5.6] Fix clearing the cache without a cache directory

[Jalle19]

Problem

As mentioned in
489c9f9, if storage_path('framework/cache') doesn't exist, php artisan cache:clear fails with the following error:

$ php artisan cache:clear -vvv

In Finder.php line 543:
                                                                        
  [InvalidArgumentException]                                            
  The "/vagrant/api/storage/framework/cache" directory does not exist.  

This happens regardless of the cache driver used, including array.

Solution

Practically all cache drivers except file work just fine without the framework/cache directory, so it's common to have deployments where said directory doesn't exist. Clearing the cache should still work, especially since an array or redis cache should have nothing to do with the filesystem.

I've added appropriate tests and took the liberty to clean the whole test case up a bit.

[Jalle19]

@taylorotwell you want me to do a backport to the 5.5 branch since this was AFAICT introduced in 5.5.6?