x/build: dragonfly-amd64 builders may need a longer TEST_TIMEOUT_SCALE

[bcmills]

I've noticed frequent cmd/go test timeouts on the dragonfly-amd64 builders recently (#45215). Some other tests tend to time out on these builders too (#34034).

I notice that the dragonfly-amd64 builders are reverse builders, so I wonder if the builders are just on the slow side. Perhaps they need a GO_TEST_TIMEOUT_SCALE to allow the more intensive tests to complete?

(See also #44584.)

CC @tuxillo @golang/release

[tuxillo]

@bcmills is that some configuration option? Let me know if you think I should change something.

[dmitshur]

@tuxillo Yes, it allows providing a custom multiplier for the default test timeout. See:

go/src/cmd/dist/test.go

Lines 149 to 161 in 189c694

	t.timeoutScale = 1
	switch goarch {
	case "arm":
	t.timeoutScale = 2
	case "mips", "mipsle", "mips64", "mips64le":
	t.timeoutScale = 4
	}
	if s := os.Getenv("GO_TEST_TIMEOUT_SCALE"); s != "" {
	t.timeoutScale, err = strconv.Atoi(s)
	if err != nil {
	fatalf("failed to parse $GO_TEST_TIMEOUT_SCALE = %q as integer: %v", s, err)
	}
	}

It's useful for cases where a builder has limited CPU/IO/network resources that result in default test timeouts being too aggressive, since it gives tests more time to finish. See golang/build@3689a13 and golang/build@3a0fcf7 for past examples where that configuration was adjusted.

[tuxillo]

Ok, I'll submit a patch to build with a timescale of 2, unless you want to set another value?