test(exproto): fix too small buffer size for listener

These options have not been picked up by the listener before esockd
5.11.0, so it was not apparent that they are not working.

apps/emqx_gateway_exproto/test/emqx_exproto_SUITE.erl

@@ -663,9 +663,11 @@ tcp_opts() ->
 
 udp_opts() ->
     #{
-        recbuf => 1024,
-        sndbuf => 1024,
-        buffer => 1024,
+        %% NOTE
+        %% Making those too small will lead to inability to accept connections.
+        recbuf => 2048,
+        sndbuf => 2048,
+        buffer => 2048,
         reuseaddr => true
     }.