[test] Assign unique MAC addresses for test network devices

[test] Assign unique MAC addresses for test network devices

Commit 19dffdc ("[efi] Allow for creating devices with no EFI parent
device") relaxed the restriction on attempting to create SNP devices
when no EFI parent device is available, with the result that the test
network devices created when running the IPv4 tests are now registered
as SNP devices.

Since the dummy EFI parent device path is fixed and the test network
device MAC addresses are empty, the SNP devices end up with identical
constructed device paths and registration of the second and subsequent
devices will fail since device paths must be unique.

Fix by assigning MAC addresses to the test network devices.

Reported-by: Miao Wang <shankerwangmiao@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>