Test that tasks will be halted within a single cron run

[brendanheywood]

• tab 1: run cron.php
• tab 2: sleep 1
• tab 2: cron.php --disable
• tab 1: cron should exit soon after

[kenneth-hendricks]

# make sure cron is on
tab 1: sudo -u www-data php admin/cli/cron.php --enable
tab 1: sudo -u www-data php admin/cli/cron.php

tab 2: sudo -u www-data php admin/cli/cron.php --disable

tab 1: should stop soon