selftests: openvswitch: add basic ct test case parsing

Forwarding via ct() action is an important use case for openvswitch, but
generally would require using a full ovs-vswitchd to get working. Add a
ct action parser for basic ct test case.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Adrian Moreno <amorenoz@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>