Don't read errno unless the syscall failed in mq_notify01 test

Don't read errno unless the syscall failed in mq_notify01 test

 I've seen failures reporting "return value=0, errno=EACCESS" in the
mq_notify01 test. This is because the syscall actually succeeded (as
expected by the test), but, as it is implemented by a library
function, some of the (more) syscalls it invoked failed. The POSIX
specification of errno says it should only be read when the call in
question failed - therefore, this patch hardcodes errno 0 when return
value is 0.

 Also, this patch removes a useless TEST macro usage in a place the
TEST_RETURN nor TEST_ERRNO variables are never checked.