cmd/cover: fix missing error check in test

Fix missing error check in test.

Spotted by Gordon Klass, https://groups.google.com/forum/#!topic/golang-nuts/MdDLbvOjb4o

Change-Id: I22f1a438cbb60a2fe1740fc2d43fbf8aa008b6de
Reviewed-on: https://go-review.googlesource.com/10605Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cover/cover_test.go

@@ -59,7 +59,9 @@ func TestCover(t *testing.T) {
 	for i, line := range lines {
 		lines[i] = bytes.Replace(line, []byte("LINE"), []byte(fmt.Sprint(i+1)), -1)
 	}
-	err = ioutil.WriteFile(coverInput, bytes.Join(lines, []byte("\n")), 0666)
+	if err := ioutil.WriteFile(coverInput, bytes.Join(lines, []byte("\n")), 0666); err != nil {
+		t.Fatal(err)
+	}
 
 	// defer removal of test_line.go
 	if !debug {