Add merge trains checkbox selector and update tests

ee/app/views/projects/_merge_trains_settings.html.haml

 - return unless merge_trains_available?
 
 .form-check
-  = form.check_box :merge_trains_enabled, class: 'form-check-input'
+  = form.check_box :merge_trains_enabled, class: 'form-check-input', data: { qa_selector: 'merge_trains_checkbox' }
   = form.label :merge_trains_enabled, class: 'form-check-label' do
     = s_('ProjectSettings|Enable merge trains.')
     .text-secondary.mb-2

qa/qa/ee/page/project/settings/merge_request.rb

@@ -17,6 +17,10 @@ module QA
                 view 'ee/app/views/projects/_merge_pipelines_settings.html.haml' do
                   element :merged_results_pipeline_checkbox
                 end
+
+                view 'ee/app/views/projects/_merge_trains_settings.html.haml' do
+                  element :merge_trains_checkbox
+                end
               end
             end
 
@@ -24,8 +28,13 @@ module QA
               check_element :merged_results_pipeline_checkbox
             end
 
+            def click_merge_trains_checkbox
+              check_element :merge_trains_checkbox
+            end
+
             def enable_merge_train
               click_pipelines_for_merged_results_checkbox
+              click_merge_trains_checkbox
               click_save_changes
             end
           end

qa/qa/specs/features/ee/browser_ui/6_release/pipelines_for_merged_results_and_merge_trains_spec.rb

@@ -50,6 +50,7 @@ module QA
         Page::Project::Settings::Main.perform do |main|
           main.expand_merge_requests_settings do |settings|
             settings.click_pipelines_for_merged_results_checkbox
+            settings.click_merge_trains_checkbox
             settings.click_save_changes
           end
         end