Add tests for unset output marks on floating policies

This change adds tests to ensure backwards compatibility of floating
policies with no output mark set, ensuring that they are still routed
correctly (over the default network) in mark-based routing schemes.

Bug: 122236988
Test: Ran on common-4.9, passing
Change-Id: I49d291ad5b6a51bc4556643c96e37885c49dce5d