We attempt to mimic the behaviour of Intel's PXE ROM by skipping the
separate ProxyDHCPREQUEST if the ProxyDHCPOFFER already contains a
boot filename or a PXE boot menu.

Experimentation reveals that Intel's PXE ROM will also check for a
non-empty next-server address alongside the boot filename.  Update our
test to match this behaviour.

Reported-by: Wissam Shoukair <wissams@mellanox.com>
Tested-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>