devlink: Move devlink dev selftest code to dev

Move devlink dev selftest callbacks and related code from leftover.c to
file dev.c. No functional change in this patch.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>