Nightly time source failures on several platforms

[clalancette]

Several of the time source tests failed on the nightlies, though the number of failing tests varied from platform to platform.

One example is https://ci.ros2.org/view/nightly/job/nightly_linux-aarch64_debug/816/testReport/junit/(root)/projectroot/test_time_source_2/ (which is the only failing time source on Linux aarch64).

Another example is https://ci.ros2.org/view/nightly/job/nightly_osx_debug/1251/testReport/junit/rclcpp/TestTimeSource/clock/ , where several different tests failed on that nightly job.

[clalancette]

This one seems to be fixed in the latest nightlies, so closing.