Test Updating OUTPUT_MARK on Active SAs

Test that the xfrm output mark is update-able
on an Active SA, which means that we can dynamically
reroute traffic to new underlying networks after an
SA has been created.

Bug: 71645364
Test: run_net_test.sh xfrm_test.py
Change-Id: I561fdc27439d33807667c4a58a03bca3c468599b