Direct Transfer - Extend E2Es to ensure group items are migrated correctly

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Currently our nightly tests for Direct Transfer migrate projects only: https://gitlab.com/gitlab-org/manage/import/import-metrics:

Screenshot from a recent run posted in #import-export-metrics

This is problematic because it won't catch issues related to migrated group items. We should extend our specs by adding a test case to ensure that group items (such as Epics) are correctly migrated