[7.x] Add tests for Stringable. (#31690)

* Add tests for Stringable.

* Fix StyleCI.

Author: lucasmichot

src/Illuminate/Support/Stringable.php

@@ -348,7 +348,7 @@ public function matchAll($pattern)
      */
     public function parseCallback($default = null)
     {
-        return Str::parseCallback($this->value);
+        return Str::parseCallback($this->value, $default);
     }
 
     /**