Refactor Gitlab::Ci::Config::External::Mapper::Verifier specs

What does this MR do and why?

I moved the "when a project is missing" block under the "when files are project files" block. Also, fixed the wording in "returns an array of valid file objects" to "returns an array of file objects".

ref: !118670 (comment 1369205966) and !118670 (comment 1375516249)

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