behat: make run_check() check STDERR
behat: no-op wp_mail()
behat: fix `user generate` test
don't show progress bar if not in a TTY

this fixes 'tput: No value for $TERM and no -T specified' error
suppress curl download meter if not running interactively
behat: hide export warnings
Merge pull request #538 from wp-cli/behat-check-stderr

behat: make run_check() check STDERR