Instead of having libcryb-test provide main() and assume that the test program defines t_prepare() and t_cleanup(), have libcryb-test provide a t_main() function which the test program calls with pointers to its prepare and cleanup functions.
Add several subtraction test cases and modify some others to better
exercise the carry logic, which is far more complex for subtraction than
for addition.
Add test cases where the target is the same MPI as one or both operands.
be better to systematically pad MPIs used in all tests with garbage.
Greatly expand the number of mpi_cmp() tests to cover all possible
combinations of same or different sign, same or different msb and same
or different magnitude.
Add a t_compare_mpi() predicate which checks that two MPIs have the same
value (but not necessarily the exact same representation).
Clean up and improve existing tests.
