Unify spec/lib/banzai/filter/image_lazy_load_filter_spec.rb

What does this MR do?

We don't need to pass lib: true as in EE

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Part of &27 (closed)