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>
1 file changed