validate: Pass the actual action reference to the execute function and API enhancements

• validate: Implement Debug trait on Action

• validate: Pass the actual action reference to the execute function
  We need to be able to keep a reference to the action when we implement async action types.

  There was a bug in the test where we were using a ref to the wrong action object in the async signal which is why we didn't detect the problem.

  To make that API simple we should implement a #[validate_test] proc macro.

• validate: Fix refcount handling for action.set_done()
  Also fix the test making proper use of it.

• validate: action: Expose the .scenario() method

• validate: action: Expose the .report_error() method