Add unit tests for some struct coins functions.
Add unit tests for some struct coins functions.

Added tests for `coins_alloc_gp_cp_description()`, `coins_is_zero()`,
`coins_make()`, `coins_make_from_cp()` and `coins_make_zero()`.

donmccaughey/FiendsAndFortune,donmccaughey/FiendsAndFortune,donmccaughey/FiendsAndFortune