selftests: net: lib: support errexit with busywait

If errexit is enabled ('set -e'), loopy_wait -- or busywait and others
using it -- will stop after the first failure.

Note that if the returned status of loopy_wait is checked, and even if
errexit is enabled, Bash will not stop at the first error.

Fixes: 25ae948b

 ("selftests/net: add lib.sh")
Cc: stable@vger.kernel.org
Acked-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240605-upstream-net-20240605-selftests-net-lib-fixes-v1-1-b3afadd368c9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>