[release-branch.go1.8] cmd/go: add test for test -race -i behavior

This was fixed in CL 37598 but the test was (rightly) dropped
because it modified $GOROOT. Here's a variant that does not.

For #19151.

Change-Id: Iccdbbf9ae8ac4c252e52f4f8ff996963573c4682
Reviewed-on: https://go-review.googlesource.com/39592


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/39618


Reviewed-by: Austin Clements <austin@google.com>