tresor_tester: make command pool a proper module

* The command pool used to be kind of a module but it was driven via custom
  tresor-tester specific code. With this commit it becomes a proper module that
  is driven by the module framework instead.

* Move the code for creating and handling the module-execution progress flag
  into Module_composition::execute_modules as the function is always used with
  this code surrounding it.

Ref genodelabs#5062

repos/gems/src/app/tresor_init/main.cc

@@ -101,21 +101,24 @@ class Main
 		 */
 		void wakeup_vfs_user() override { _sigh.local_submit(); }
 
-		void _execute()
+		void _wakeup_back_end_services()
 		{
-			bool progress { true };
-			while (progress) {
-
-				progress = false;
-				execute_modules(progress);
-			}
-
 			_vfs_env.io().commit();
+		}
 
+		void _try_end_program()
+		{
 			if (_state == COMPLETE)
 				_env.parent().exit(0);
 		}
 
+		void _execute()
+		{
+			execute_modules();
+			_try_end_program();
+			_wakeup_back_end_services();
+		}
+
 		/****************
 		 ** Module API **
 		 ****************/