Use nip.io instead of non-standard .localhost tld

Quoting Wikipedia:
> The name localhost is a **commonly** defined hostname for the loopback
> interface in most TCP/IP systems, resolving to the IP addresses
> 127.0.0.1 in IPv4 and ::1 for IPv6.

test/server/lib/helpers/TestServer.ts

@@ -199,7 +199,7 @@ export class TestServer {
 
 // FIXME: found that TestProxyServer exist, what should I do? :'(
 export class TestServerProxy {
-  public static readonly HOSTNAME: string = 'grist-test-proxy.localhost';
+  public static readonly HOSTNAME: string = 'grist-test-proxy.127.0.0.1.nip.io';
 
   private _stopped: boolean = false;
   private _app = express();