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/file_test.go b/src/net/file_test.go
index d81bca7..6fab06a 100644
--- a/src/net/file_test.go
+++ b/src/net/file_test.go
@@ -89,7 +89,7 @@
 
 func TestFileListener(t *testing.T) {
 	switch runtime.GOOS {
-	case "windows":
+	case "nacl", "windows":
 		t.Skipf("skipping test on %q", runtime.GOOS)
 	}