fix: ensure id included in supervisor child spec

[gf3]

If id is not set in child_spec/1 it defaults to __MODULE__ which prevents more than one instance of HighlanderPG from running. Here is the output from the added test before the fix is applied:

  1) test can start multiple HighlanderPG instances (HighlanderPGTest)
     test/highlander_pg_test.exs:150
     ** (EXIT from #PID<0.1853.0>) bad child specification, more than one child specification has the id: HighlanderPG.
     If using maps as child specifications, make sure the :id keys are unique.
     If using a module or {module, arg} as child, use Supervisor.child_spec/2 to change the :id, for example:

         children = [
           Supervisor.child_spec({MyWorker, arg}, id: :my_worker_1),
           Supervisor.child_spec({MyWorker, arg}, id: :my_worker_2)
         ]

I also captured the log output from one of the tests to make it a little less noisy.