vet_test currently uses a custom GOPATH for each test, but it turns
out not to be necessary.

Updates #30228

Change-Id: Id7a7bf6d759bd94adccf44e197be1728c2f23575
Reviewed-on: https://go-review.googlesource.com/c/163038
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>