io/ioutil: close file in TestWriteFile before deleting it

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5495086

src/pkg/io/ioutil/ioutil_test.go

@@ -60,6 +60,7 @@ func TestWriteFile(t *testing.T) {
 	}
 
 	// cleanup
+	f.Close()
 	os.Remove(filename) // ignore error
 }