cmd/gosh: use an os.Pipe in another interactive test

The writer didn't close the pipe, so the runner would sometimes hang
waiting for the writer to be finished. Switch to os.Pipe while here too.

cmd/gosh/main_test.go

@@ -232,9 +232,13 @@ func TestInteractive(t *testing.T) {
 }
 
 func TestInteractiveExit(t *testing.T) {
-	inReader, inWriter := io.Pipe()
+	inReader, inWriter, err := os.Pipe()
+	qt.Assert(t, qt.IsNil(err))
 	defer inReader.Close()
-	go io.WriteString(inWriter, "exit\n")
+	go func() {
+		io.WriteString(inWriter, "exit\n")
+		inWriter.Close()
+	}()
 	w := io.Discard
 	runner, _ := interp.New(interp.StdIO(inReader, w, w))
 	if err := runInteractive(runner, inReader, w, w); err != nil {