Add stored_events helper to retrieve events since last command

Mark the position in the event store when executing a command. The
`stored_event` and `then_events` helpers retrieve and compare the
events stored since the last command execution.

lib/sequent/test/command_handler_helpers.rb

@@ -124,17 +124,14 @@ def given_events(*events)
           Sequent::Core::BaseCommand.new,
           to_event_streams(events.flatten(1)),
         )
-        @helpers_given_events_position_mark = Sequent.configuration.event_store.position_mark
       end
 
       def when_command(command)
+        @helpers_events_position_mark = Sequent.configuration.event_store.position_mark
         Sequent.configuration.command_service.execute_commands command
       end
 
       def then_events(*expected_events)
-        stored_events, @helpers_given_events_position_mark =
-          Sequent.configuration.event_store.load_events_since_marked_position(@helpers_given_events_position_mark)
-
         expected_classes = expected_events.flatten(1).map { |event| event.instance_of?(Class) ? event : event.class }
         expect(stored_events.map(&:class)).to eq(expected_classes)
 
@@ -159,6 +156,10 @@ def then_no_events
         then_events
       end
 
+      def stored_events
+        Sequent.configuration.event_store.load_events_since_marked_position(@helpers_events_position_mark)[0]
+      end
+
       private
 
       def to_event_streams(uncommitted_events)