appc: add test to ensure that individual IPs are not removed during route updates

If control advised the connector to advertise a route that had already
been discovered by DNS it would be incorrectly removed. Now those routes
are preserved.

Updates tailscale/corp#16833

Signed-off-by: James Tucker <james@tailscale.com>

appc/appconnector.go

@@ -155,7 +155,7 @@ nextRoute:
 	for _, r := range routes {
 		for _, addr := range e.domains {
 			for _, a := range addr {
-				if r.Contains(a) {
+				if r.Contains(a) && netip.PrefixFrom(a, a.BitLen()) != r {
 					pfx := netip.PrefixFrom(a, a.BitLen())
 					toRemove = append(toRemove, pfx)
 					continue nextRoute