cmd/go: fail tests immediately if they attempt to create a tempfile within GOROOT

cmd/go: fail tests immediately if they attempt to create a tempfile within GOROOT

This will help to detect regressions of #28387 when running
'go test cmd/go' in a writable GOROOT.

Updates #28387
Updates #30316

Change-Id: I551e044111535404688b1a76e63163dfcb41bb5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/207701
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>