Adjust TestOnlyLoopbackExistsWhenUsingDisableNetworkOption to ignore "DOWN" interfaces

This fixes the following, which I've been seeing on all my machines for as long as I can remember:

--- FAIL: TestOnlyLoopbackExistsWhenUsingDisableNetworkOption (0.36 seconds)
	container_test.go:1597: Wrong interface count in test container: expected [*: lo], got [1: lo 2: sit0]

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

integration/container_test.go

@@ -1553,7 +1553,7 @@ func TestOnlyLoopbackExistsWhenUsingDisableNetworkOption(t *testing.T) {
 	runtime := mkRuntimeFromEngine(eng, t)
 	defer nuke(runtime)
 
-	config, hc, _, err := runconfig.Parse([]string{"-n=false", GetTestImage(runtime).ID, "ip", "addr", "show"}, nil)
+	config, hc, _, err := runconfig.Parse([]string{"-n=false", GetTestImage(runtime).ID, "ip", "addr", "show", "up"}, nil)
 	if err != nil {
 		t.Fatal(err)
 	}