public async Task StartTracking_Should_start_listening_for_changes()
{
_permissionValidator.ValidateGeolocationPermission().Returns(true);
_geolocator.IsListening.Returns(false);
await _trackerService.StartTracking();
await _geolocator.Received(1).StartListeningAsync(Arg.Any <TimeSpan>(), Arg.Any <double>());
_geolocator.Received(1).PositionChanged += Arg.Any <EventHandler <PositionEventArgs> >();
}