code queries

[Bran S]

I have seen tail_alloc() being used at a lot of places in tests. For
example, in ioctl_hdio.c, and epoll_pwait.c.

File tests.h gives information about what it does but I don't
understand the motive and use of allocating memory in such a manner.
What is the purpose of it? How is it helpful in testing?

Another query is about error_msg.c file:
Link: https://github.com/strace/strace/blob/master/tests/error_msg.c#L9

In tests/error_msg.c:
       #define perror_msg_and_fail perror_msg_and_fail
       #define error_msg_and_fail error_msg_and_fail

What is the reason for doing this?

Thanks
Bran