os/signal: wait for goroutine in TestTerminalSignal

Fixes #28169

Change-Id: I187d9effea56357bbb04d4971d284a52ffae61f8
Reviewed-on: https://go-review.googlesource.com/c/142889
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/signal/signal_cgo_test.go

@@ -22,6 +22,7 @@ import (
 	"os/signal/internal/pty"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 	"testing"
 	"time"
@@ -113,7 +114,11 @@ func TestTerminalSignal(t *testing.T) {
 	const prompt = "prompt> "
 
 	// Read data from master in the background.
+	var wg sync.WaitGroup
+	wg.Add(1)
+	defer wg.Wait()
 	go func() {
+		defer wg.Done()
 		input := bufio.NewReader(master)
 		var line, handled []byte
 		for {