selftests/net: test l2 tunnel TOS/TTL inheriting

There are currently 3 ip tunnels that are capable of carrying
L2 traffic: gretap, vxlan and geneve.
They all are capable to inherit the TOS/TTL for the outer
IP-header from the inner frame.

Add a test that verifies that these fields are correctly inherited.

These tests failed before the following commits:
b09ab9c9 ("ip6_tunnel: allow to inherit from VLAN encapsulated IP")
3f8a8447 ("ip6_gre: use actual protocol to select xmit")
41337f52 ("ip6_gre: set DSCP for non-IP")
7ae29fd1 ("ip_tunnel: allow to inherit from VLAN encapsulated IP")
7074732c ("ip_tunnels: allow VXLAN/GENEVE to inherit TOS/TTL from VLAN")
ca2bb695 ("geneve: do not use RT_TOS for IPv6 flowlabel")
b4ab94d6

 ("geneve: fix TOS inheriting for ipv4")

Signed-off-by: Matthias May <matthias.may@westermo.com>
Link: https://lore.kernel.org/r/20220817073649.26117-1-matthias.may@westermo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>