Hardcoded group name in install script

I was reading through the install script and noticed a line using the hardcoded group name "testingadmin" rather than the $TESTING_ADMIN_GROUP variable:

# Ensure the Drupal root's parent directory exists, and that the testing
# administrators can write to it.
echo "Ensuring webroot"
mkdir -p $WEB_ROOT
chmod 775 $WEB_ROOT
chgrp testingadmin $WEB_ROOT