test named remote-servers `key` usage

test named remote-servers `key` usage

Even though `remote-servers` now allows using named server-list with `key`
(or `tls`), the `key` or `tls` is not used, in the context of a named
server-list, when configuring the server.

For instance,

   remote-servers foo { 10.53.0.5; };
   also-notify { foo key fookey; };

won't use `fookey`.

Add a system test highlighting the problem.