tests.pxe: Update test steps info

Update test steps following our test plan.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
tests.pxe: Add a subtest of pxe

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>