MDL-35325 Fix qbehaviour_adaptive_walkthrough_test::test_adaptive_numerical

This was always broken, but recent coverage by ktemkin revealed it. This
fixes the problem that was causing the new coverage in the test to fail.

./vendor/bin/phpunit --filter test_adaptive_numerical$ \
    qbehaviour_adaptive_walkthrough_test question \
    /behaviour/adaptive/tests/walkthrough_test.php