Re-enable tests when intra process manager takes an allocator again

[wjwwood]

The test was disabled in #84 and it needed to be disabled because the refactored intra-process manager in rclcpp unfortunately did not have the ability to control allocations in its merged state, see:

ros2/rclcpp#778

However, the plan is update that implementation soon to again take an allocator and allow for complete control over all allocations. At that time, the disabled test should be re-enabled.

Also, at the same time it would be a good idea to refactor this test to use https://github.com/osrf/osrf_testing_tools_cpp#memory_tools instead of a custom new and delete as that approach has limitations. Also to consider how it may not work correctly for all rmw implementations, see:

#73