Verify rmw implementation of rclcpp examples at runtime

[dhood]

Modifies existing rclcpp_examples tests to act as a regression test for fda5938 (in comparison to #105 which adds new tests to explicitly check the linking)

Currently just for talker - once approved I can extend to the other executables

Note that I had to modify the exit handler used in the tests in order to get them to fail with the non-zero return code from talker (when fda5938 is reverted). This does not seem to have any negative side-effects (no regressions in CI). As a result, I am not entirely clear on the reason it was ignore_exit_handler before, given that it suppresses errors and seems to have been unnecessary. So, please be extra skeptical of this change in this PR.

http://ci.ros2.org/job/ci_windows/1265/
http://ci.ros2.org/job/ci_linux/1218/
http://ci.ros2.org/job/ci_osx/983/

Connects to #96

[wjwwood]

This is the kind of thing that I didn't want to have in the examples. (I think it detracts from the otherwise simple example code)

I was thinking we could instead put the ability to check this into the rmw library itself. I never prototyped this, but I'm still interested in doing it.

If we merge this as-is, I'd want a TODO to remind us that there is probably a way to do it which doesn't affect the user code here.

[wjwwood]

@dhood see: ros2/rcl#51

[dhood]

thanks - I'll integrate it

[dirk-thomas]

Before our executable did not exit cleanly when they received a SIGINT. Therefore the tests needed to ignore their exit code using the ignore_exit_handler. Maybe that has changed in the mean time.

[tfoote]

@dhood "should we have a macro that checks this for all tests which call it?": no, just opt in where we think it is needed.