libtest_alloc should overallocate and fill the extra data with random
bytes and store a checksum which libtest_free should use to determine
if and out of bounds write has been performed. This requires
libtest_alloc only sets the unusable memory size to what has been
required (rounded up to the next page size multiple for pvalloc). It
may also be a good idea to allocate a few bytes before the returned
address.