Remove method redefined warnings for test suite

has_many definitions with "name" as singular and as plural e.g.
  has_many :welcome_posts_with_comment
  has_many :welcome_posts_with_comments

Ruby mentions it with:

lib/active_record/associations/builder/collection_association.rb:65:
  warning: method redefined; discarding old welcome_posts_with_comment_ids
lib/active_record/associations/builder/collection_association.rb:65:
  warning: previous definition of welcome_posts_with_comment_ids was here
lib/active_record/associations/builder/collection_association.rb:75:
  warning: method redefined; discarding old welcome_posts_with_comment_ids=
lib/active_record/associations/builder/collection_association.rb:75:
  warning: previous definition of welcome_posts_with_comment_ids= was here
Merge pull request #13598 from zirni/remove_ar_warnings

Remove method redefined warnings for test suite

Closes #13526