How do I mock app() in a framework unit test

I'm working on a pull request for attribute casting in eloquent and am running into a scenario where a unit test can't resolve a class out of app() and I'm unsure how to mock it. How can I mock app()?

Fatal error: Call to a member function getBindings() on null in /Users/bkuhl/Personal/framework/src/Illuminate/Database/Eloquent/Model.php on line 2825

    protected function castAttribute($key, $value)
    {
        $castType = $this->getCastType($key);

        // Some objects aren't stored as json so we'll skip those
        // here so they'll get handled accordingly later
        if (!in_array($castType, ['date', 'datetime'])) {
            if (class_exists($this->casts[$key])) {
                return $this->resolveCastedAttribute($this->casts[$key], $value);
            }

            if (in_array($key, app()->getBindings())) {
                return $this->resolveCastedAttribute($key, $value);
            }
        }

        if (is_null($value)) {
            return $value;
        }

        switch ($castType) {
            case 'int':
            case 'integer':
                return (int) $value;
            case 'real':
            case 'float':
            case 'double':
                return (float) $value;
            case 'string':
                return (string) $value;
            case 'bool':
            case 'boolean':
                return (bool) $value;
            case 'object':
                return $this->fromJson($value, true);
            case 'array':
            case 'json':
                return $this->fromJson($value);
            case 'collection':
                return new BaseCollection($this->fromJson($value));
            case 'date':
            case 'datetime':
                return $this->asDateTime($value);
            case 'timestamp':
                return $this->asTimeStamp($value);
        }
    }

The source is here: https://github.com/bkuhl/framework/commit/b57d8ebbe3da80e49be6d310b08cc09c565b7ecc