nixos/osquery: add test

Some time ago I fixed the broken package `osquery` (see #39336).
I had to test the package manually by starting the daemon locally,
however this doesn't ensure that the module is still functional.

In order to cover the package *and* the integration with the NixOS
module I thought that adding a testcase might be the best idea.

The current testcase does the following things:

* Starts an `osqueryd` service in a test machine with customized logger
  path and PID file

* Ensures that the `osqueryd.service` unit is running

* Checks if the customized flags (`pidfile`, `logger_path`) are applied
  to `osquery`.

* Performs a simple test query against the `etc_hosts` database to check
  if the basic funcitonality of `osquery` (storing system information into
  a database) works fine.