refactor: Make update ancestors spec more maintainable

- Use correct progress terminology in test description.
- Merge tests changing work and remaining work: the tests setup and
  expectations where the same. Only the action differed: one was calling
  the service with `estimated_hours` being modified, the other with
  `remaining_hours` being modified.
- Make tests faster (from 13 secs to 5 secs)
- Introduce `shared_let_work_packages` to create work packages with
  table helpers in a `shared_let` fashion.
- Introduce `for_each_context` which runs the same set of tests with
  different RSpec shared contexts.