selftests: openvswitch: add a simple test for tunnel metadata

selftests: openvswitch: add a simple test for tunnel metadata

This test ensures that upon receiving decapsulated packets from a
tunnel interface in openvswitch, the tunnel metadata fields are
properly populated.  This partially covers interoperability of the
kernel tunnel ports and openvswitch tunnels (LWT) and parsing and
formatting of the tunnel metadata fields of the openvswitch netlink
uAPI.  Doing so, this test also ensures that fields and flags are
properly extracted during decapsulation by the tunnel core code,
serving as a regression test for the previously fixed issue with the
DF bit not being extracted from the outer IP header.

The ovs-dpctl.py script already supports all that is necessary for
the tunnel ports for this test, so we only need to adjust the
ovs_add_if() function to pass the '-t' port type argument in order
to be able to create tunnel ports in the openvswitch datapath.

Reviewed-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Link: https://patch.msgid.link/20250909165440.229890-3-i.maximets@ovn.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>