Use timestamp in group name in ee_epics_milestone_dates_spec.rb

What does this MR do?

Use a timestamp in the group name in an E2E test so that the janitor job on staging can clean up the group when it's old.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/quality/team-tasks/issues/65#note_127306365

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [-] Security reports checked/validated by reviewer