NearestNeighborTest: use ts->get_rng() instead of (implicit) theRNG()
NearestNeighborTest: use ts->get_rng() instead of (implicit) theRNG()

This ensures that test data is not dependent on the order the tests are
executed in.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv