Add FIPS spec helper

What does this MR do and why?

Describe in detail what your merge request does and why.

Extracts this spec helper from !81917 (merged) (see that MR for examples of use) as it's generally useful for FIPS work and that MR is taking a while to finish.

This allows you to tag a spec group with :fips_mode and have it run with Gitlab::FIPS.enabled? returning true. It does this by manipulating the env var used for detection.

Related #325172 (closed)

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