Fix double tap implementation

[Russell-Newton]

Currently, double tap never fails

Known Issues:

• double tap uses ints for x, y, t
• setting the x, y, and t of the double tap data to match those of the stroke data every update means dist and delta_t are always 0
• double.h still has that coords struct that we don't want to use

Additionally, create some unit tests that ensure failure

Acceptance Criteria:

• double tap fails when we expect it to