io/ioutil: Fix dropped test error

Change-Id: I9cfaba4f1af23ab67627bf496739311e4d1984c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/205245Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/io/ioutil/ioutil_test.go

@@ -72,6 +72,9 @@ func TestReadOnlyWriteFile(t *testing.T) {
 
 	// We don't want to use TempFile directly, since that opens a file for us as 0600.
 	tempDir, err := TempDir("", t.Name())
+	if err != nil {
+		t.Fatalf("TempDir %s: %v", t.Name(), err)
+	}
 	defer os.RemoveAll(tempDir)
 	filename := filepath.Join(tempDir, "blurp.txt")