Update epic migration sync specs

What does this MR do and why?

Adds group/namespace id check and iid check to epic migration sync tests.

Also fixes position of WorkItemWidgetColor by searching for widget type instead of relying on position.

Related to gitlab-org/quality/quality-engineering/team-tasks#2651 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

This test is set to only run on staging. From gitlab/qa run:

EPIC_SYNC_TEST=true GITLAB_QA_USER_AGENT=<1password> GITLAB_USERNAME="gitlab-qa" GITLAB_PASSWORD=<1password> bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- qa/specs/features/ee/api/2_plan/epics_to_work_items_sync_spec.rb