net/netcheck: use dnscache.Resolver when resolving DERP IPs

This also adds a bunch of tests for this function to ensure that we're
returning the proper IP(s) in all cases.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0d9d57170dbab5f2bf07abdf78ecd17e0e635399

cmd/tailscale/cli/netcheck.go

@@ -48,6 +48,7 @@ func runNetcheck(ctx context.Context, args []string) error {
 	c := &netcheck.Client{
 		UDPBindAddr: envknob.String("TS_DEBUG_NETCHECK_UDP_BIND"),
 		PortMapper:  portmapper.NewClient(logger.WithPrefix(log.Printf, "portmap: "), nil, nil),
+		UseDNSCache: false, // always resolve, don't cache
 	}
 	if netcheckArgs.verbose {
 		c.Logf = logger.WithPrefix(log.Printf, "netcheck: ")