Add ex_slop, configure it, run it and fix issues

test/support/conn_case.ex

@@ -1,19 +1,5 @@
 defmodule MusicLibraryWeb.ConnCase do
-  @moduledoc """
-  This module defines the test case to be used by
-  tests that require setting up a connection.
-
-  Such tests rely on `Phoenix.ConnTest` and also
-  import other functionality to make it easier
-  to build common data structures and query the data layer.
-
-  Finally, if the test case interacts with the database,
-  we enable the SQL sandbox, so changes done to the database
-  are reverted at the end of every test. If you are using
-  PostgreSQL, you can even run database tests asynchronously
-  by setting `use MusicLibraryWeb.ConnCase, async: true`, although
-  this option is not recommended for other databases.
-  """
+  @moduledoc false
 
   use ExUnit.CaseTemplate