Reactively verify File caches are flushed

This change subtly adjusts how directory deletions are confirmed. The main change being that we're reactively confirming if the directory was deleted.

The original code assumes that the deleteDirectory method is always going to return false when it fails. This is an incorrect assumption to make.
Because the deleteDirectory method actually will only return false if the input path isn't a directory. And it will silently fail if it has issues with removal of the directory and/or it's children items.

Author: mallardduck

src/Illuminate/Cache/FileStore.php

@@ -179,7 +179,8 @@ public function flush()
         }
 
         foreach ($this->files->directories($this->directory) as $directory) {
-            if (! $this->files->deleteDirectory($directory)) {
+            $results = $this->files->deleteDirectory($directory);
+            if (! $results || $this->files->exists($directory)) {
                 return false;
             }
         }