Avoid lossage from <stdnoreturn.h>
A few of the source files in tests/ns
included <isc/util.h>
before <cmocka.h>
. This could cause compile failures because the
CMOCKA_NORETURN
macro is defined as __attribute__((noreturn))
and <stdnoreturn.h>
defines noreturn
as _Noreturn
which does
not work as a gcc-style attribute.