Fix acceptance test assumption about exec logging.

The acceptance test for overriding the agent name fact was broken, in that it
assumed that exec { "echo foo": } would always write foo to the output
channel.

This isn't true - on success, that output was simply hidden. By requesting
that output be logged we can ensure that message will show up and fix the
problem.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>