DscpPolicyTest: use tap instead of tun

This switches the DscpPolicyTest to use a tap interface rather than a
tun interface. This should now test the same bpf code path that is used
in production and the raw ip version should be able to be removed.

Test: atest DscpPolicyTest
Bug: 243891394
Bug: 243120975
Change-Id: I1633b85810838afde6191f0a5094269e45c15ae4