test: eth: Add a test for the target being pinged

test: eth: Add a test for the target being pinged

The target will respond to pings while doing other network handling.
Make sure that the response happens and is correct.

This currently corrupts the ongoing operation of the device if it
happens to be awaiting an ARP reply of its own to whatever serverip it
is attempting to communicate with. In the test, add an expectation that
the user operation (ping, in this case) will fail. A later patch will
address this problem.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>