test_get_user_notification_preferences() unit test fails with third-party message providers

There is a new unit test in 3.2 - core_message_externallib_testcase::test_get_user_notification_preferences() - that makes assertions on the number of components providing notifications to a user. This fails when there is a third-party plugin installed that has a message provider that is accessible to users by default.

To replicate:

1. install the attached plugin and initialise phpunit
2. Run vendor/bin/phpunit --filter=test_get_user_notification_preferences core_message_externallib_testcase message/tests/externallib_test.php
3. You should see a failure with the message "Failed asserting that actual size 9 matches expected size 8."