Follow up - Generate stages with label information from fixtures

Summary

In #32112 (closed) we introduced a fixture spec to generate test data for the cycle analytics tests, but there is currently no stage data with labels being seeded. To get around this, we manually mutate a start and stop event to be of type 'label'.

When the cycle analytics backend is complete, we should be able to remove this manual mutation

Related to: !19361 (merged)

Improvements

All test data generated from a fixture spec

Involved components

ee/spec/frontend/analytics/cycle_analytics/mock_data.js

Optional: Missing test coverage