test: use current expirationd api

Commit "Big refactoring with API redo and gh-6-10"
(41fb516) introduced updated API
methods: all methods were renamed and changed interface for
expirationd.start() (previously expirationd.run_task()). However TAP
tests still continue to test old version of API.

Updated method's names:

- run_task() -> start()
- task_stats() -> stats()
- kill_task() -> kill()
- get_task() -> task()
- show_task_list() -> tasks()