net: don't run multicast listen test on nil interface in short mode

The gccgo bug report https://gcc.gnu.org/PR65785 points out that the
multicast listen tests will use the network even with -test.short.
Fix test by checking testing.Short with a nil interface.

Change-Id: I7eab8df34fe3b78fc376912312fac9d0f94977f1
Reviewed-on: https://go-review.googlesource.com/17154Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/listen_test.go

@@ -542,7 +542,7 @@ func TestIPv4MulticastListener(t *testing.T) {
 		// routing stuff for finding out an appropriate
 		// nexthop containing both network and link layer
 		// adjacencies.
-		if ifi == nil && !*testExternal {
+		if ifi == nil && (testing.Short() || !*testExternal) {
 			continue
 		}
 		for _, tt := range ipv4MulticastListenerTests {
@@ -618,7 +618,7 @@ func TestIPv6MulticastListener(t *testing.T) {
 		// routing stuff for finding out an appropriate
 		// nexthop containing both network and link layer
 		// adjacencies.
-		if ifi == nil && (!*testExternal || !*testIPv6) {
+		if ifi == nil && (testing.Short() || !*testExternal || !*testIPv6) {
 			continue
 		}
 		for _, tt := range ipv6MulticastListenerTests {