Create dynamic frontend test fixture for releases

The following discussion from !19226 (merged) should be addressed:

• @mikegreiling started a discussion: (+1 comment)

  I suppose if our mock_data was generated from backend samples instead of being a static fixture our tests might have caught this property name discrepancy.

  @winh is there a good way to generate these things on-the-fly? Should we create a follow-up issue?

spec/frontend/releases/mock_data.js should be replaced by a dynamic fixture. see also https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#frontend-test-fixtures