Fix libnetwork/osl test TestAddRemoveInterface

For some time, when adding an interface with no IPv6 address (an
interface to a network that does not have IPv6 enabled), we've been
disabling IPv6 on that interface.

As part of a separate change, I'm removing that logic - there's nothing
wrong with having IPv6 enabled on an interface with no routable address.
The difference is that the kernel will assign a link-local address.

TestAddRemoveInterface does this...
- Assign an IPv6 link-local address to one end of a veth interface, and
  add it to a namespace.
- Add a bridge with no assigned IPv6 address to the namespace.
- Remove the veth interface from the namespace.
- Put the veth interface back into the namespace, still with an
  explicitly assigned IPv6 link local address.

When IPv6 is disabled on the bridge interface, the test passes.

But, when IPv6 is enabled, the bridge gets a kernel assigned link-local
address.

Then, when re-adding the veth interface, the test generates an error in
'osl/interface_linux.go:checkRouteConflict()'. The conflict is between
the explicitly assigned fe80::2 on the veth, and a route for fe80::/64
belonging to the bridge.

So, in preparation for not-disabling IPv6 on these interfaces, use a
unique-local address in the test instead of link-local.

I don't think that changes the intent of the test.

With the change to not-always disable IPv6, it is possible to repro the
problem with a real container, disconnect and re-connect a user-defined
network with '--subnet fe80::/64' while the container's connected to an
IPv4 network. So, strictly speaking, that will be a regression.

But, it's also possible to repro the problem in master, by disconnecting
and re-connecting the fe80::/64 network while another IPv6 network is
connected. So, I don't think it's a problem we need to address, perhaps
other than by prohibiting '--subnet fe80::/64'.

Signed-off-by: Rob Murray <rob.murray@docker.com>

libnetwork/osl/sandbox_linux_test.go

@@ -72,7 +72,7 @@ func newInfo(t *testing.T, hnd *netlink.Handle) (*Namespace, error) {
 	}
 	addr.IP = ip4
 
-	ip6, addrv6, err := net.ParseCIDR("fe80::2/64")
+	ip6, addrv6, err := net.ParseCIDR("fdac:97b4:dbcc::2/64")
 	if err != nil {
 		return nil, err
 	}
@@ -116,7 +116,7 @@ func newInfo(t *testing.T, hnd *netlink.Handle) (*Namespace, error) {
 	return &Namespace{
 		iFaces: []*Interface{intf1, intf2, intf3},
 		gw:     net.ParseIP("192.168.1.1"),
-		gwv6:   net.ParseIP("fe80::1"),
+		gwv6:   net.ParseIP("fdac:97b4:dbcc::1/64"),
 	}, nil
 }