Follow up - Migrate mock data to fixture spec

Summary

We are introducing some mock data in !15216 (merged), the backend is currently being implement in !16240 (merged). When the backend is merged, we should generate the mock data from a fixture spec as per our frontend testing guide

Improvements

JSON data generated from the backend, less hardcoded mock data.

Risks

Involved components

• ee/app/assets/javascripts/analytics/cycle_analytics/components/custom_stage_form.vue

Optional: Missing test coverage

Tests already exist using the current mock data, so this should be a drop in replacement