use a kernel determined server address in proxy tests

Now we can execute the proxy tests in parallel without running into
"address already in use" errors.

integrationtests/tools/proxy/proxy.go

@@ -75,7 +75,10 @@ type QuicProxy struct {
 }
 
 // NewQuicProxy creates a new UDP proxy
-func NewQuicProxy(local string, version protocol.VersionNumber, opts Opts) (*QuicProxy, error) {
+func NewQuicProxy(local string, version protocol.VersionNumber, opts *Opts) (*QuicProxy, error) {
+	if opts == nil {
+		opts = &Opts{}
+	}
 	laddr, err := net.ResolveUDPAddr("udp", local)
 	if err != nil {
 		return nil, err