Refactor listing files to be independent of exiting runners

Previously, we were always fetching files, even if not required (e.g., for score runs). Now, we reduce the number of file listings and use a dedicated callback.

app/controllers/submissions_controller.rb

@@ -178,7 +178,7 @@ class SubmissionsController < ApplicationController
         send_and_store client_socket, message
       end
 
-      runner_socket.on :exit do |exit_code, files|
+      runner_socket.on :exit do |exit_code|
         @testrun[:exit_code] = exit_code
         exit_statement =
           if @testrun[:output].empty? && exit_code.zero?
@@ -201,6 +201,10 @@ class SubmissionsController < ApplicationController
           @testrun[:status] = :out_of_memory
         end
 
+        # The client connection will be closed once the file listing finished.
+      end
+
+      runner_socket.on :files do |files|
         downloadable_files, = convert_files_json_to_files files
         if downloadable_files.present?
           js_tree = FileTree.new(downloadable_files).to_js_tree