The method you are calling on HelperRS is a static method, whereas you are creating a mocked instance of that class and not actually doing anything with it. You can achieve this with mockery, though it is not recommended and will require you to run the test in process isolation.
$mock = Mockery::mock('alias:HelperRS');
$mock->shouldReceive('getRecibosLocacao')->once()->andReturn('mocked');