nixos/graylog: add test

Basic test which confirms new inputs can be created and that messages
can be sent to a UDP-GELF input using `netcat`.

This test requires 4GB of RAM to avoid issues due insufficient
memory (please refer to `nixos/tests/elk.nix` for a detailed explanation of
the issue) for elasticsearch.

Also it's ensured that elasticsearch has an open HTTP port for communication
when starting `graylog`. This is a workaround to ensure that all services
are started in proper order, even in test environments with less power.
However this shouldn't be implemented in the `nixos/graylog` module as
this might be harmful when using elasticsearch clusters that require e.g.
authentication and/or run on different servers.