net: separate NaCl dependent placeholders from BSD's

To clarify the dependency of NaCl platform.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/143830044
diff --git a/src/net/dial_test.go b/src/net/dial_test.go
index 19e289f..abeb500 100644
--- a/src/net/dial_test.go
+++ b/src/net/dial_test.go
@@ -465,6 +465,11 @@
 }
 
 func TestDialDualStackLocalhost(t *testing.T) {
+	switch runtime.GOOS {
+	case "nacl":
+		t.Skipf("skipping test on %q", runtime.GOOS)
+	}
+
 	if ips, err := LookupIP("localhost"); err != nil {
 		t.Fatalf("LookupIP failed: %v", err)
 	} else if len(ips) < 2 || !supportsIPv4 || !supportsIPv6 {