os/signal: fix t.Fatal that should be t.Fatalf

Change-Id: I2a24b2bde9a7c641b3bc802ff8b2ddebf4990109
Reviewed-on: https://go-review.googlesource.com/79496
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/signal/signal_cgo_test.go

@@ -149,7 +149,7 @@ func TestTerminalSignal(t *testing.T) {
 
 	// Set the bash prompt so that we can see it.
 	if _, err := master.Write([]byte("PS1='" + prompt + "'\n")); err != nil {
-		t.Fatal("setting prompt: %v", err)
+		t.Fatalf("setting prompt: %v", err)
 	}
 	select {
 	case <-sawPrompt: