Laravel IoC on helper class unit test

I'm creating a helper class to work with Filesystem, that gets injected via Laravel IoC like this:

// MyHelperClass
public function __construct(Filesystem $filesystem) {
    $this->filesystem = $filesystem;
}

The problem is I have no idea, nor I can find any documentation on how to instantiate MyHelperClass inside a TestCase subclass. I tried IoC'ing it in the __construct(MyHelperClass $myHelperClass) and setUp(MyHelperClass $myHelperClass) methods but that didn't work