proxmoxer: disable flaky test (#425411)

pkgs/development/python-modules/proxmoxer/default.nix

···
   disabledTests = [
     # Tests require openssh_wrapper which is outdated and not available
     "test_repr_openssh"
+
+    # Test fails randomly
+    "test_timeout"
   ];
 
   pythonImportsCheck = [ "proxmoxer" ];