tstest/natlab: use net.ErrClosed

We are now on 1.16.
And wgconn.NetErrClosed has been removed upstream.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tstest/natlab/natlab.go

@@ -26,7 +26,6 @@ import (
 	"sync"
 	"time"
 
-	wgconn "github.com/tailscale/wireguard-go/conn"
 	"inet.af/netaddr"
 )
 
@@ -759,8 +758,7 @@ func (c *conn) canRead() error {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if c.closed {
-		// TODO: when we switch to Go 1.16, replace this with net.ErrClosed
+		return net.ErrClosed
-		return wgconn.NetErrClosed
 	}
 	if !c.readDeadline.IsZero() && c.readDeadline.Before(time.Now()) {
 		return errors.New("read deadline exceeded")