Add test for incrementing with negative number
Add test for incrementing with negative number

It is now possible to increment with negative numbers, but no tests for
this existed

Signed-off-by: Kim Christensen <aa8e4f65ee1f7dee724d3c54a33e387cc279d580@gmail.com>

mvno/Okanshi,mvno/Okanshi,mvno/Okanshi