selftests/net: l2_tos_ttl_inherit.sh: Set IPv6 addresses with "nodad".

The ping command can run before DAD completes. In that case, ping may
fail and break the selftest.

We don't need DAD here since we're working on isolated device pairs.

Fixes: b690842d

 ("selftests/net: test l2 tunnel TOS/TTL inheriting")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>