net: fix conf.teardown call in TestGoLookupIPOrderFallbackToFile

If the test fails, conf.teardown wouldn't be.
It doesn't look like it matters much, but clean up anyway.

Change-Id: I45c18095abfd49422975d061be20cbd971a98f8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/180780
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix_test.go

@@ -589,6 +589,8 @@ func TestGoLookupIPOrderFallbackToFile(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer conf.teardown()
+
 	if err := conf.writeAndUpdate([]string{}); err != nil {
 		t.Fatal(err)
 	}
@@ -620,7 +622,6 @@ func TestGoLookupIPOrderFallbackToFile(t *testing.T) {
 			t.Errorf("%s: address doesn't match expectation. got %v, want %v", name, got, want)
 		}
 	}
-	defer conf.teardown()
 }
 
 // Issue 12712.