Fix some cases where CC knobs could be detected as select knobs.

* Use 0x00 as a tell that the control is not a select knob.
* If there are 8 or more distinct values, do not map a select knob.

Eight is arbitrary but you have to really crank on most select knobs to produce
values outside of the range of +1/-1 so I think this is fine. We may see jog
wheels that break this rule though.

Added tests for these confusable cases. Also fixed an old test.