tests: Add export_one and latest helpers to the Task test interface
tests: Add export_one and latest helpers to the Task test interface

- The 'export_one' method will export one task matching a filter,
  raising error if not exactly one task was matched.

- The 'latest' property will return the task matching the "+LATEST"
  filter.

tbabej/task,tbabej/task,tbabej/task,tbabej/task,tbabej/task