selftests: openvswitch: Add support for tunnel() key.

This will be used when setting details about the tunnel to use as
transport.  There is a difference between the ODP format between tunnel():
the 'key' flag is not actually a flag field, so we don't support it in the
same way that the vswitchd userspace supports displaying it.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240625172245.233874-5-aconole@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>