Remove the validate! method tests from CI include files

What does this MR do and why?

In !108826 (merged), we introduced the ci_batch_request_for_local_and_project_includes feature flag and deprecate the validate! method of external-files. However, we are still using them in tests. In this MR, we are replacing it with Gitlab::Ci::Config::External::Mapper::Verifier.new.process because it runs every validation according to the order.

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