refactor: elevator closure header/footer handling

* Rather than the main Elevator screen candidate generator creating the
  header and footer widgets and the Closures generator "modifying" them
  with variants as appropriate, now the Closures generator alone is
  responsible. This somewhat simplifies the implementation and testing
  on both ends.

* Tests for the Closures generator were more verbose and testing more
  than they strictly needed to be (for example, that the correct header
  and footer were present in every case). These are now cut down to only
  the relevant and unique details.

lib/screens/v2/candidate_generator/elevator.ex

@@ -5,12 +5,15 @@ defmodule Screens.V2.CandidateGenerator.Elevator do
   alias Screens.V2.CandidateGenerator.Elevator.Closures, as: ElevatorClosures
   alias Screens.V2.CandidateGenerator.Widgets.Evergreen
   alias Screens.V2.Template.Builder
-  alias Screens.V2.WidgetInstance.{Footer, NormalHeader}
-  alias ScreensConfig.Screen
-  alias ScreensConfig.V2.Elevator
 
   @behaviour CandidateGenerator
 
+  @instance_fns [
+    &ElevatorClosures.elevator_status_instances/2,
+    &Evergreen.evergreen_content_instances/2
+  ]
+
+  @impl true
   def screen_template do
     {
       :screen,
@@ -26,29 +29,11 @@ defmodule Screens.V2.CandidateGenerator.Elevator do
     |> Builder.build_template()
   end
 
-  def candidate_instances(
-        config,
-        now \\ DateTime.utc_now(),
-        elevator_closure_instances_fn \\ &ElevatorClosures.elevator_status_instances/3,
-        evergreen_content_instances_fn \\ &Evergreen.evergreen_content_instances/2
-      ) do
-    Enum.concat([
-      elevator_closure_instances_fn.(
-        config,
-        header_instance(config, now),
-        footer_instance(config)
-      ),
-      evergreen_content_instances_fn.(config, now)
-    ])
+  @impl true
+  def candidate_instances(config, now \\ DateTime.utc_now(), instance_fns \\ @instance_fns) do
+    instance_fns |> Enum.map(& &1.(config, now)) |> Enum.concat()
   end
 
+  @impl true
   def audio_only_instances(_widgets, _config), do: []
-
-  defp header_instance(%Screen{app_params: %Elevator{elevator_id: elevator_id}} = config, now) do
-    %NormalHeader{text: "Elevator #{elevator_id}", screen: config, time: now}
-  end
-
-  defp footer_instance(config) do
-    %Footer{screen: config}
-  end
 end

lib/screens/v2/candidate_generator/elevator/closures.ex

@@ -31,6 +31,7 @@ defmodule Screens.V2.CandidateGenerator.Elevator.Closures do
   alias Screens.Routes.Route
   alias Screens.Stops.Stop
   alias Screens.V2.WidgetInstance
+  alias Screens.V2.WidgetInstance.{Footer, NormalHeader}
 
   alias Screens.V2.WidgetInstance.{
     CurrentElevatorClosed,
@@ -54,12 +55,10 @@ defmodule Screens.V2.CandidateGenerator.Elevator.Closures do
 
   @fallback_summary "Visit mbta.com/elevators for more info"
 
-  @spec elevator_status_instances(Screen.t(), NormalHeader.t(), Footer.t()) ::
-          list(WidgetInstance.t())
+  @spec elevator_status_instances(Screen.t(), DateTime.t()) :: list(WidgetInstance.t())
   def elevator_status_instances(
-        %Screen{app_params: %ElevatorConfig{elevator_id: elevator_id} = app_params},
-        header_instance,
-        footer_instance
+        %Screen{app_params: %ElevatorConfig{elevator_id: elevator_id} = app_params} = config,
+        now
       ) do
     {:ok, alerts} = @alert.fetch_elevator_alerts_with_facilities()
 
@@ -68,17 +67,27 @@ defmodule Screens.V2.CandidateGenerator.Elevator.Closures do
 
     case Enum.find(closures, fn %Closure{id: id} -> id == elevator_id end) do
       nil ->
-        [header_instance, elevator_closures_list(closures, app_params), footer_instance]
+        [elevator_closures_list(closures, app_params) | header_footer_instances(config, now)]
 
       _closure ->
         [
-          %NormalHeader{header_instance | variant: :closed},
-          %CurrentElevatorClosed{app_params: app_params},
-          %Footer{footer_instance | variant: :closed}
+          %CurrentElevatorClosed{app_params: app_params}
+          | header_footer_instances(config, now, :closed)
         ]
     end
   end
 
+  defp header_footer_instances(
+         %Screen{app_params: %ElevatorConfig{elevator_id: elevator_id}} = config,
+         now,
+         variant \\ nil
+       ) do
+    [
+      %NormalHeader{text: "Elevator #{elevator_id}", screen: config, time: now, variant: variant},
+      %Footer{screen: config, variant: variant}
+    ]
+  end
+
   defp elevator_closure(%Alert{id: id, effect: :elevator_closure, informed_entities: entities}) do
     # We expect there is a 1:1 relationship between `elevator_closure` alerts and individual
     # out-of-service elevators. Log a warning if our assumptions don't hold.