os/signal: increase settle time in tests

I noticed a timeout in TestIgnore in
https://build.golang.org/log/52d83a72f3a5ea9a16eb5d670c729694144f9624,
which suggests that the settle time is currently set too low.

I've also added a check for the same GO_TEST_TIMEOUT_SCALE used in
TestTerminalSignal, so that if this builder remains too slow we can
increase the builder's scale factor rather than the test's baseline
running time.

Updates #33174

Change-Id: I18b10eaa3bb5ae2f604300aedaaf6f79ee7ad567
Reviewed-on: https://go-review.googlesource.com/c/go/+/227649
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/signal/signal_test.go

@@ -25,16 +25,30 @@ import (
 // settleTime is an upper bound on how long we expect signals to take to be
 // delivered. Lower values make the test faster, but also flakier — especially
 // on heavily loaded systems.
-const settleTime = 100 * time.Millisecond
+//
+// The current value is set based on flakes observed in the Go builders.
+var settleTime = 250 * time.Millisecond
+
+func init() {
+	if s := os.Getenv("GO_TEST_TIMEOUT_SCALE"); s != "" {
+		if scale, err := strconv.Atoi(s); err == nil {
+			settleTime *= time.Duration(scale)
+		}
+	}
+}
 
 func waitSig(t *testing.T, c <-chan os.Signal, sig os.Signal) {
+	t.Helper()
 	waitSig1(t, c, sig, false)
 }
 func waitSigAll(t *testing.T, c <-chan os.Signal, sig os.Signal) {
+	t.Helper()
 	waitSig1(t, c, sig, true)
 }
 
 func waitSig1(t *testing.T, c <-chan os.Signal, sig os.Signal, all bool) {
+	t.Helper()
+
 	// Sleep multiple times to give the kernel more tries to
 	// deliver the signal.
 	start := time.Now()
@@ -58,7 +72,7 @@ func waitSig1(t *testing.T, c <-chan os.Signal, sig os.Signal, all bool) {
 			timer.Reset(settleTime / 10)
 		}
 	}
-	t.Fatalf("timeout waiting for %v", sig)
+	t.Fatalf("timeout after %v waiting for %v", settleTime, sig)
 }
 
 // quiesce waits until we can be reasonably confident that all pending signals