RuboCop: Include helpers only in related specs

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Problem

In #382438 (closed) showed a conflict between RuboCop's parse_source spec helper and the subject definition in spec/lib/gitlab/ci/parsers/sbom/cyclonedx_properties_spec.rb.

!103847 (merged) fixed this issue and @minac suggested:

Actually, a better solution would be to just include Rubocop helpers for rubocop related specs but it would require more time so here is a quick fix.

Proposed solution

Include CopHelper only in RuboCop related specs.

Refs

Follow-up of !103847 (comment 1170079223) and #382438 (closed)