os/signal: increase test timeouts on ppc64*/linux

TestSignalTrace can take a bit longer to run on some
ppc64{,le} machines. I was only able to reproduce the
timeout reliably on a POWER8/ppc64le ubuntu 16.04 host.

Bump the timeout to 5 seconds.  This should be more than
sufficient for a test which occasionally takes a bit
longer to run on these builders.

Fixes #45773

Change-Id: I4177bb986561f714aabfa5b0ca44b1c90b1cd94f
Reviewed-on: https://go-review.googlesource.com/c/go/+/315049
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>

src/os/signal/signal_test.go

@@ -18,6 +18,7 @@ import (
 	"runtime"
 	"runtime/trace"
 	"strconv"
+	"strings"
 	"sync"
 	"syscall"
 	"testing"
@@ -47,6 +48,13 @@ func init() {
 		//
 		// See https://golang.org/issue/33174.
 		settleTime = 11 * time.Second
+	} else if runtime.GOOS == "linux" && strings.HasPrefix(runtime.GOARCH, "ppc64") {
+		// Older linux kernels seem to have some hiccups delivering the signal
+		// in a timely manner on ppc64 and ppc64le. When running on a
+		// ppc64le/ubuntu 16.04/linux 4.4 host the time can vary quite
+		// substantially even on a idle system. 5 seconds is twice any value
+		// observed when running 10000 tests on such a system.
+		settleTime = 5 * time.Second
 	} else if s := os.Getenv("GO_TEST_TIMEOUT_SCALE"); s != "" {
 		if scale, err := strconv.Atoi(s); err == nil {
 			settleTime *= time.Duration(scale)