I finally reproduced the famous unknown error issue I've been hearing about and nobody knew where it came from.

Steps to reproduce the problem:

1. Install and enable your behat test site: php admin/tool/behat/cli/init.php
2. Enable your behat test site: php admin/tool/behat/cli/util.php --enable
3. You see the message; ok, this is enabled, with the command to copy & paste, this is ok, is useful when you add new steps for example
4. Install your behat test site again using util.php: php admin/tool/behat/cli/util.php --install nobody should do it as it does not make sense, but who know, I've just done it by mistake
5. It will say that the tables are already present and we can not install the site BUT it before that it already reset the dataroot dir.
6. Try to enable the test environment again
7. You will see an unknown error Unknown error 1 This is not a behat test site!
8. Try to run the tests
9. You will see Your behat test site is outdated, please run php admin/tool/behat/cli/init.php from your moodle dirroot to drop and install the behat test site again.

The only solution to all this is to --drop and --install again.

We should check that the site is not installed before resetting the dataroot dir.