Add specs for adding-to/clear has_many collections’s behavior on `updated_at`

There are behaviors mentioned in #17161 that:

1. are not documented properly, and
2. don't have specs

This commit addresses the spec absence. For has_many collections,

1. addition (<<) should update the associated object's updated_at (if any)
2. .clear, depending on options[:dependent], calls delete_all, destroy_all, or nullifies the associated object(s)' foreign key.