Fix broke CE tests due to merge train pipeline check

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52137 introduced a
`merge_train_pipeline?` check in the view, but this method was only
available in EE.

We now define a method in CE that is overriden in EE.

app/models/ci/pipeline.rb

@@ -1214,6 +1214,11 @@ module Ci
     end
     # rubocop:enable Rails/FindEach
 
+    # EE-only
+    def merge_train_pipeline?
+      false
+    end
+
     private
 
     def add_message(severity, content)

ee/app/models/ee/ci/pipeline.rb

@@ -158,6 +158,7 @@ module EE
         end
       end
 
+      override :merge_train_pipeline?
       def merge_train_pipeline?
         merge_request_pipeline? && merge_train_ref?
       end