Use agnhost instead of busybox for network test (#9390)

busybox container requires a root permission for ping.
For testing hardening method at CI, we need to switch to another image
which doesn't require the root permission for network testing.
On kubernetes/kubernetes repo, we are using agnhost which doesn't
require it. So this makes the test use aghhost image.

In addition, this updates the test manifest to specify securityContext
without any privilege.