Import JSON fixtures as modules

What does this MR do and why?

Change loading of JSON fixtures from reading file to requiring as module. This would allow Jest to track dependency on JSON fixtures, paving way for further work related to #339343 (closed)

Screenshots or screen recordings

Before:

$ jest --config jest.config.js --listTests --findRelatedTests tmp/tests/frontend/fixtures-ee/api/freeze-periods/freeze_periods.json

Done in 15.44s.

After:

$ jest --config jest.config.js --listTests --findRelatedTests tmp/tests/frontend/fixtures-ee/api/freeze-periods/freeze_periods.json
spec/frontend/deploy_freeze/components/timezone_dropdown_spec.js
spec/frontend/deploy_freeze/components/deploy_freeze_settings_spec.js
spec/frontend/deploy_freeze/components/deploy_freeze_modal_spec.js
spec/frontend/deploy_freeze/store/actions_spec.js
spec/frontend/deploy_freeze/components/deploy_freeze_table_spec.js
spec/frontend/deploy_freeze/store/mutations_spec.js
Done in 19.39s.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #339346 (closed)