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.
a caller-provided buffer. Use it to warn about leaks in each individual
test case. Note that we can't fail a test case for leaking, because
individual test cases in a unit may modify shared state which is cleaned
up at the end of the series.
