I asked for more tests as part of MDL-53713 changes and woolardfa@appstate.edu pointed me to https://github.com/moodle/moodle/blob/master/lib/classes/session/manager.php#L135 which seems to be blocking all other session handlers from being while running unit tests. I couldn't find any reason in MDL-31501 for it but the result, if I am not missing something, is that we don't test other session handlers than the file one, lib/tests/session_manager_test.php seems to confirm this. Would be nice to do it. MDL-53599 landed in 3.1 and MDL-53713 is another example of why they would be useful.