net: deflake TestLookupNonLDH

Fixes #17623.

Change-Id: I4717e8399f955c9be7ba19108bb0bcc108187c04
Reviewed-on: https://go-review.googlesource.com/32147
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup_test.go

@@ -722,6 +722,10 @@ func TestLookupNonLDH(t *testing.T) {
 	if runtime.GOOS == "nacl" {
 		t.Skip("skip on nacl")
 	}
+	if fixup := forceGoDNS(); fixup != nil {
+		defer fixup()
+	}
+
 	// "LDH" stands for letters, digits, and hyphens and is the usual
 	// description of standard DNS names.
 	// This test is checking that other kinds of names are reported