net: reenable TestLookupLongTXT

This test was disabled for flakiness.
The underlying code has been rewritten.
Reenabling the test to see if it has been fixed.

Fixes #22857.

Change-Id: If488ee5f4692cdf4ed534b8f08b4a54b80a60943
Reviewed-on: https://go-review.googlesource.com/c/go/+/423454
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Damien Neil <dneil@google.com>

src/net/lookup_test.go

@@ -416,7 +416,6 @@ func TestLookupGoogleHost(t *testing.T) {
 }
 
 func TestLookupLongTXT(t *testing.T) {
-	testenv.SkipFlaky(t, 22857)
 	mustHaveExternalNetwork(t)
 
 	defer dnsWaitGroup.Wait()