No idea if the technique was original, it’s been too long, but I wrote about it on the Cloudflare blog in 2016. https://blog.cloudflare.com/go-coverage-with-external-tests/

Anyway, very happy to see the team ship first class support for it. It immediately made coverage support in testscript much more natural, too. https://github.com/rogpeppe/go-internal/pull/201