Fix FIPS tests for dependency scanning tests

What does this MR do and why?

Previously these tests were failing on a FIPS system (https://gitlab.com/gitlab-org/gitlab/-/jobs/26692419830). Fix this by properly stubbing the tests so that FIPS mode is explicitly disabled or enabled.

Screenshots or screen recordings

Relates to #364562 (closed)

How to set up and validate locally

FIPS_MODE=true bundle exec rspec ee/spec/lib/gitlab/ci/templates/dependency_scanning_gitlab_ci_yaml_spec.rb

MR acceptance checklist

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