Unlike assert(3), which uses abort(3), this has no other side effects (before raising SIGABRT) than an fprintf() call. The test framework will catch the SIGABRT, report that the test case failed, and proceed with the next case.
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.