Add reload to epic work item sync spec

What does this MR do and why?

Updates the epic to work items sync spec to wait for the new milestone dates since they're updated in a sidekiq job.

Related to 7bd533bc

Note: This is a preemptive fix. There hasn't been a failure in the pipeline that runs this spec YET but I've encountered this when running the test locally and against staging while working on other things today.

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

Numbered steps to set up and validate the change are strongly suggested.

Testing against staging:

1. From the gitlab/qa folder, 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

Testing locally:

1. In rails console, enable work item epics

   Feature.enable(:work_item_epics)
   Feature.enable(:work_item_epics_rollout)
   Feature.enable(:work_items_rolledup_dates)

2. Then, from the gitlab/qa folder, run:

   EPIC_SYNC_TEST=true \
   bundle exec bin/qa Test::Instance::All http://gdk.test:3000 -- qa/specs/features/ee/api/2_plan/epics_to_work_items_sync_spec.rb