Execution of CRM_Core_BAO_ActionScheduleTest breaks CiviTest/CiviMailUtil.php

CRM_Core_BAO_ActionScheduleTest, CRM_Mailing_BAO_SpoolTest, and CRM_Core_Payment_BaseIPNTest perform mock email delivery, and each test passes individually. However, when run as part of CRM_AllTests, the ActionScheduleTest appears to break the SpoolTest and BaseIPNTest.

ActionScheduleTest uses an older approach to mocking mail delivery; the other two classes use a helper, CiviTest/CiviMailUtils.php, to handle mocking. These appear to be incompatible.

To reproduce, delete all *Test.php files from tests/phpunit except for the three involved tests – and then run CRM_AllTests.