selftests: move timestamping selftests to net folder

For historical reasons, there are several timestamping selftest targets
in selftests/networking/timestamping. Move them to the standard
directory for networking tests: selftests/net.

Signed-off-by: Jian Yang <jianyang@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>