internal/testenv: add a test for the GoTool function

internal/testenv: add a test for the GoTool function

GoTool was added in CL 20967, and revised in CL 21292, for #14901.

I don't fully understand what problem the GoTool function was added to
solve: the discussion on that issue was pretty sparse, but it seems
like when we run tests of GOROOT packages they always know their own
location relative to GOROOT (and thus always know where to find the
'go' tool).

Lacking that understanding, I don't want to change its behavior, but I
do at least want to verify that it resolves to the real 'go' tool in
the common case (running 'go test' on a package in GOROOT/src).

For #50892
For #50893
Updates #14901

Change-Id: I06d831e6765be631dfc4854d7fddc3d27fc1de34
Reviewed-on: https://go-review.googlesource.com/c/go/+/381834
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>