After the last Moodle release the plugin tests fail. That happens because some new event has been added and tests do not consider that.

vendor/bin/phpunit mod/hsuforum/tests/mail_test.php

Moodle 3.5.4 (Build: 20190114), a0d236fbd777133b7aa063564eeec08540f4f221
Php: 7.1.24.1.16.04.1.1, pgsql: 9.5.14, OS: Linux 4.4.0-134-generic x86_64
PHPUnit 6.5.7 by Sebastian Bergmann and contributors.

F...................... 23 / 23 (100%)

Time: 51.86 seconds, Memory: 104.00MB

There was 1 failure:

1) mod_hsuforum_mail_testcase::test_cron_message_includes_courseid
Failed asserting that null matches expected '2'.

/var/www/project/monash35/mod/hsuforum/tests/mail_test.php:287
/var/www/project/monash35/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
vendor/bin/phpunit "mod_hsuforum_mail_testcase" mod/hsuforum/tests/mail_test.php

FAILURES!
Tests: 23, Assertions: 227, Failures: 1.