kv rest api: add remaining range tests

Another way to verify would be to construct expected results. You might do that via a Scan of the underlying rows directly from the server's db. That way you could set up a list of tests which scan with different start, end and limit keys and then compare the results using !reflect.DeepEqual(results, expResults).

Not necessary to do this--I think these tests are fine. But something to keep in mind.