wgengine: shut down wireguard on Close

This was (presumably) missing from wgengine because the
interactions between magicsock and wireguard-go meant that the
shutdown never worked. Now those are fixed, actually shut down.

Fixes occasional flake in expanded ipn/e2e_test.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

wgengine/userspace.go

@@ -575,6 +575,7 @@ func (e *userspaceEngine) Close() {
 
 	r := bufio.NewReader(strings.NewReader(""))
 	e.wgdev.IpcSetOperation(r)
+	e.wgdev.Close()
 	e.linkMon.Close()
 	e.router.Close()
 	e.magicConn.Close()