net: give more detail when test panics

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11811043

src/pkg/net/interface_linux_test.go

@@ -78,7 +78,7 @@ var (
 func TestParseProcNet(t *testing.T) {
 	defer func() {
 		if p := recover(); p != nil {
-			t.Fatalf("panicked")
+			t.Fatalf("parseProcNetIGMP or parseProtNetIGMP6 panicked: %v", p)
 		}
 	}()
 

src/pkg/net/unicast_posix_test.go

@@ -436,8 +436,8 @@ func TestWildWildcardListener(t *testing.T) {
 	}
 
 	defer func() {
-		if recover() != nil {
-			t.Fatalf("panicked")
+		if p := recover(); p != nil {
+			t.Fatalf("Listen, ListenPacket or protocol-specific Listen panicked: %v", p)
 		}
 	}()