Reservoir Sampling vs Round Robin
it seems that you're asking why bother with reservoir sampling when you're capable of tricking the test that you wrote? 

Round robin doesn't return random numbers. It returns numbers deterministically. Well much more deterministic seeming than reservoir/other methods.

Your tests should be better. If you need the result to seem random and not deterministic, make a test which captures that instead of one which is based on empirical probability.

EDIT to add a test example:
another test for randomness (one that round robin would fail) is that you run the same process many times and don't get the same result every time. For uniform random sampling of a stream (the output of reservoir sampling) for a set/list/stream of a fixed size the probability to get one specific subset of size k should be 1/(n choose k) . You can run your method once then another 10000 (or any number of times) and see that you get the first result approximately only 1/(n choose k) times.