os/signal: deflake TestReset/TestIgnore

Fixes #15661.

Change-Id: Ic3a8296fc7107f491880900ef52563e52caca1a3
Reviewed-on: https://go-review.googlesource.com/23615
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/signal/signal_test.go

@@ -139,6 +139,19 @@ func testCancel(t *testing.T, ignore bool) {
 		Reset(syscall.SIGWINCH, syscall.SIGHUP)
 	}
 
+	// At this point we do not expect any further signals on c1.
+	// However, it is just barely possible that the initial SIGWINCH
+	// at the start of this function was delivered after we called
+	// Notify on c1. In that case the waitSig for SIGWINCH may have
+	// picked up that initial SIGWINCH, and the second SIGWINCH may
+	// then have been delivered on the channel. This sequence of events
+	// may have caused issue 15661.
+	// So, read any possible signal from the channel now.
+	select {
+	case <-c1:
+	default:
+	}
+
 	// Send this process a SIGWINCH. It should be ignored.
 	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)