Deprecation of `gnrc_nettest`

Hi there,

Jose and I proposed to deprecate gnrc_nettest [1]. It was originally introduced to allow for integration tests of single GNRC modules, but it never really got used and it is most often easier to just use gnrc_netreg_register()/unregister() directly for that.

In the very unlikely case that anybody is using it, please speak up now!

Best regards, Martine