test(integration): Use epic IID instead of ID in DeleteEpic cleanup.

The DeleteEpic function requires the epic's internal ID (IID), scoped per group, not the global ID.

This bug went undetected because only a single epic was ever created across all integration tests. With exactly one epic in the system, its global ID and its group-scoped IID are both 1 — so passing the wrong field had no observable effect.

Adding TestCreateWorkItem in !2751, which creates a work item of type Epic, incremented the global epic ID counter to 2 independently of the IID sequence. The epic created by TestEpicTimeFieldParsing then received IID 1 within its group but global ID 2. The cleanup function passed global ID 2 to DeleteEpic, which the API rejected with 403 Forbidden since no epic with IID 2 existed in that group.