WIP: Enable Ruby JIT for the test suite

What does this MR do?

Enables Ruby's JIT for the test suite

Per https://developer.squareup.com/blog/rubys-new-jit/ , this might be a terrible idea at the moment. I just want to see whether:

1. The test suite actually runs
2. How performance compares to a master test suite run

Switching to ruby 2.7 brings along a number of JIT improvements: https://medium.com/@k0kubun/jit-development-progress-at-ruby-2-7-d6dd62a8c76a - if things are close, we might find that a ruby bump is enough to make this worthwhile.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team