We're seeing these in our CI for Moodle 4.0

1) core_login_notifications_testcase::test_mobile_app_login_notificationError: Call to undefined function external_generate_token_for_current_user()
/var/www/html/login/tests/notifications_test.php:158/var/www/html/lib/phpunit/classes/advanced_testcase.php:80
2) core_login_notifications_testcase::test_mobile_app_login_notification_skip_same_ipError: Call to undefined function external_generate_token_for_current_user()
/var/www/html/login/tests/notifications_test.php:207/var/www/html/lib/phpunit/classes/advanced_testcase.php:80
3) core_login_notifications_testcase::test_mobile_app_login_notification_skip_same_deviceError: Call to undefined function external_generate_token_for_current_user()
/var/www/html/login/tests/notifications_test.php:254/var/www/html/lib/phpunit/classes/advanced_testcase.php:80
ERRORS!

I suspect it's reproduceable by doing an init call and then running the test immediately after - probably similar to WP-2774 (we run our tests in suites here)