add basic DNAT/SNAT rule default tests

* DRY out tests accordingly
Add tests confirming defaults for SNAT/DNAT

Ensure we test the 'full rule' case for defaults, to ensure that
we are testing that additional fields (such as 'Protocol' in the SNAT case'
are not appearing unexpectedly.
Add SNAT specific "disabled rule" test

and rename DNAT test for clarity
Add UDP DNAT test
Fix uncovered DNAT protocol bug

UDP test highlighted that we were not handling "protocol" correctly