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)
Merge pull request #4880 from tianon/ignore-down-interfaces

Adjust TestOnlyLoopbackExistsWhenUsingDisableNetworkOption to ignore "DOWN" interfaces