Add tests to add/remove/re-add a NAT.

This is in preparation of work to support multiple upstreams.

Test: netd_unit_test still passes with current behavior.
Bug: 38218697
Bug: 64382985
Bug: 64976379
Bug: 64995262
Change-Id: I16bdb69fa151c72d77663025bed82259f34a9608
1 file changed