Test weak nodes failing since Mar 13, 2018

[mikaelarguedas]

While it was flaky before, it looks that it got worse recently
Last night testWeakNodes failed on various platforms:
https://ci.ros2.org/view/nightly/job/nightly_win_rel/751/
https://ci.ros2.org/view/nightly/job/nightly_win_deb/813/
https://ci.ros2.org/view/nightly/job/nightly_osx_release/783/
https://ci.ros2.org/view/nightly/job/nightly_osx_debug/785/

Only changes since yesterday that seem relevant is:
ros2/rclcpp#431 (@wjwwood is it possible that this PR caused the failure?)

There was also a change inf Fast-RTPS but I don't think it's the cause:
eProsima/Fast-DDS@3514b09

[clalancette]

Running it under valgrind showed me this: https://gist.github.com/clalancette/92c4a2a0a3c396b339e839ed5fe4753c

[wjwwood]

Ok, sorry guys I saw that test and thought it was flaky, but I guess I was too hasty. Let me revert that change and bring this regression to the contributors attention.