testutils: use unicast MACs in simple_eth_pkt

This "simple" function shouldn't create multicast packets. Changed to use the
same MACs as the other packet creation functions.

Reported by Tony van der Peet <tony.vanderpeet@gmail.com>.
Merge into master from pull request #84:
testutils: use unicast MACs in simple_eth_pkt (https://github.com/floodlight/oftest/pull/84)