Switch to github import path for testify

Summary:
Debian only provides a package for the GitHub version so it's either
this or we'd have to maintain a patch to change the import paths at
packaging, a moving target for sure.

Since it's only a development dependency, we're not too worried about
versioning it and this is a much more simple solution.

Refs T175609

Test Plan: Run go get ./... && go test ./....

Reviewers: thcipriani, Release-Engineering-Team

Reviewed By: thcipriani, Release-Engineering-Team

Tags: Release-Engineering-Team

Maniphest Tasks: T175609

Differential Revision: https://phabricator.wikimedia.org/D782