Just a note on this thread that the 2 remaining tests that had problems (a- and *a) are now fixed this was caused by a change in PHP 7.x for the eval function : before PHP 7.x if the expression to evaluate had a syntax error, the eval function just returned false. Now it throws an exception and stops the script.
Including this line in a try { } catch block resolved the issue.
See https://moodle.org/mod/forum/discuss.php?d=366549 for the details