Remove rust-lang/libtest-next

This was moved under rust-lang before T-testing-devex was aware of the
crate ownership policy.
This is an experiment that we see being viable for long term support but
we are unsure if we'd want to do that within the project or not.
Considering the burden to Expatriate a package once published, we are
moving the repo out of rust-lang so we can publish them and have the
flexibility of having it maintained outside the project or later inside
the project without having to abandon the name.

See
[zulip](https://rust-lang.zulipchat.com/#narrow/channel/404371-t-testing-devex/topic/crate.20ownership/near/513724212)
for the discussion and decision.

Author: epage