internal/testenv: add GoTool

internal/testenv: add GoTool

GoToolPath requires a *testing.T to handle errors.
GoTool provides a variant that returns errors
for clients without a *testing.T,
such as that found in CL 27811.

Change-Id: I7ac8b7ec9d472894c37223c5f7b121ec823e7f61
Reviewed-on: https://go-review.googlesource.com/28787
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>