Move _SetInboundMarking to multinetwork_base.py.

This will allow us to call _SetInboundMarking in tests that do
not inherit from InboundMarkingTest, such as xfrm_tunnel_test.py.
While we're at it, move InboundMarkingTest as well, so the code
that uses _SetInboundMarking lives next to it.

Bug: 70371070
Test: all_tests.sh passes on android-4.9
Change-Id: Ief089c199aa7f6a68f1aa3c163080e1bf6bf07e7