The test was failing with core-image-full-cmdline which uses sleep from coreutils instead of sleep from busybox.
(From OE-Core rev: 8497edf0c56da34ea7b7509b8fcd46fcba2fd0e3)
Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>