internal/gen: allow tests to pass if GOPATH is not set This may make it easier to pass tests alternative build setups. Change-Id: Ib0465792e198261031449536f615200e55e2be0e Reviewed-on: https://go-review.googlesource.com/31392 TryBot-Result: Gobot Gobot <gobot@golang.org> Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>