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>

src/cmd/go/go_test.go

@@ -683,8 +683,11 @@ func (tg *testgoData) creatingTemp(path string) {
 	// If we have changed the working directory, make sure we have
 	// an absolute path, because we are going to change directory
 	// back before we remove the temporary.
-	if tg.wd != "" && !filepath.IsAbs(path) {
-		path = filepath.Join(tg.pwd(), path)
+	if !filepath.IsAbs(path) {
+		if tg.wd == "" || strings.HasPrefix(tg.wd, testGOROOT) {
+			tg.t.Fatalf("internal testsuite error: creatingTemp(%q) within GOROOT/src", path)
+		}
+		path = filepath.Join(tg.wd, path)
 	}
 	tg.must(robustio.RemoveAll(path))
 	tg.temps = append(tg.temps, path)