networkd currently silently accepts some strings as MAC addresses that it
probably shouldn't (like "ab💿ef:12:34:56:78" and "ab💿ef:12:3 4:56").
Add tests to MAC address parsing to ensure that we only accept valid MAC
addresses, and that we accept the three most common forms of MAC address
(colon-delimited hex, IEEE, and Cisco)
Several of these tests currently fail, but another commit in this series will
resolve them.
