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.
allocation failures which are either harmless (e.g. failing to allocate
the test description string) or will trigger a segfault which the driver
now catches, allowing subequent test cases to run.
