This seems to be a reoccurring flaky test, as a test introduced in #383 has been relaxed a couple of times (#501, and #907), but the test may still fail on different platforms. The measured period between timer callbacks is supposed to be 0.1s, but the linux test result below shows that the period could be less than half of that. While the test is not meant to measure timer jitter, the results suggest that the timer jitter cannot be reliably bounded.

See:
https://ci.ros2.org/view/nightly/job/nightly_linux_repeated/1780/testReport/(root)/projectroot/test_multi_threaded_executor/
https://ci.ros2.org/view/nightly/job/nightly_osx_repeated/1866/testReport/(root)/projectroot/test_multi_threaded_executor/
https://ci.ros2.org/view/nightly/job/nightly_windows-container_repeated/6/testReport/(root)/projectroot/test_multi_threaded_executor/