net: give more detail when test panics

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11811043
diff --git a/src/pkg/net/interface_linux_test.go b/src/pkg/net/interface_linux_test.go
index 085d3de..d8800bd 100644
--- a/src/pkg/net/interface_linux_test.go
+++ b/src/pkg/net/interface_linux_test.go
@@ -78,7 +78,7 @@
 func TestParseProcNet(t *testing.T) {
 	defer func() {
 		if p := recover(); p != nil {
-			t.Fatalf("panicked")
+			t.Fatalf("parseProcNetIGMP or parseProtNetIGMP6 panicked: %v", p)
 		}
 	}()