os/signal: increase wait time for signal delivery time in testcase

This increases the time to wait from 1 to 2 seconds in the
TestAtomicStop testcase. When running with gccgo on ppc64
& ppc64le on a loaded systems these testcases can
intermittently fail with the current value.

Updates #29046

Change-Id: If420274dd65926d933a3024903b5c757c300bd60
Reviewed-on: https://go-review.googlesource.com/c/153826
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/signal/signal_test.go

@@ -432,12 +432,12 @@ func atomicStopTestProgram() {
 
 		// At this point we should either die from SIGINT or
 		// get a notification on cs. If neither happens, we
-		// dropped the signal. Give it a second to deliver,
-		// which is far far longer than it should require.
+		// dropped the signal. It is given 2 seconds to
+		// deliver, as needed for gccgo on some loaded test systems.
 
 		select {
 		case <-cs:
-		case <-time.After(1 * time.Second):
+		case <-time.After(2 * time.Second):
 			if !printed {
 				fmt.Print("lost signal on tries:")
 				printed = true