Add fixture for generating mock data for Epic tree

Summary

We've introduced Epic Tree as a part of https://gitlab.com/gitlab-org/gitlab-ee/issues/10795, currently behind feature flag :epic_trees.

Frontend tests for the Vuex app currently use hand-written mock data instead of one generated using fixture. We need to generate the mock data using the same Graph query as the one used in the app to make the tests maintainable in the long run.