net: enable builtin dns resolver tests on solaris

LGTM=aram
R=golang-codereviews, aram
CC=golang-codereviews
https://golang.org/cl/99890045
diff --git a/src/pkg/net/dnsconfig_unix_test.go b/src/pkg/net/dnsconfig_unix_test.go
index 697c69f..37ed493 100644
--- a/src/pkg/net/dnsconfig_unix_test.go
+++ b/src/pkg/net/dnsconfig_unix_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package net