cmd/dist: allow $GO_TEST_TIMEOUT_SCALE to override timeoutScale

Some machines are so slow that even with the default timeoutScale,
they still timeout some tests. For example, currently some linux/arm
builders and the openbsd/arm builder are timing out the runtime
test and CL 8397 was proposed to skip some tests on openbsd/arm
to fix the build.

Instead of increasing timeoutScale or skipping tests, this CL
introduces an environment variable $GO_TEST_TIMEOUT_SCALE that
could be set to manually set a larger timeoutScale for those
machines/builders.

Fixes #10314.

Change-Id: I16c9a9eb980d6a63309e4cacd79eee2fe05769ee
Reviewed-on: https://go-review.googlesource.com/9223Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -98,6 +98,12 @@ func (t *tester) run() {
 	if t.goarch == "arm" || t.goos == "windows" {
 		t.timeoutScale = 2
 	}
+	if s := os.Getenv("GO_TEST_TIMEOUT_SCALE"); s != "" {
+		t.timeoutScale, err = strconv.Atoi(s)
+		if err != nil {
+			log.Fatalf("failed to parse $GO_TEST_TIMEOUT_SCALE = %q as integer: %v", s, err)
+		}
+	}
 
 	if t.runRxStr != "" {
 		t.runRx = regexp.MustCompile(t.runRxStr)