do not attempt to execute base tests

The OF 1.3 match.MatchTest class was intended to just be a base class, but
since it inherits from unittest.TestCase it was being run as a test.

This change ignores test classes that don't include a runTest field.
match-1.3: add tests for masked vlan VID/PCP

Masking vlan PCP is not required by OpenFlow, but switches are allowed to
implement it.