behat: remove "it should run without errors" step

introduce "try `cmd`" steps.
add success message for `wp theme delete`
don't create a new test dir if one already exists

this allows having nested WP installs
behat: fix theme test
behat: misc fixes
behat: move theme dir acquiring to the 'Given' block
behat: more misc fixes
behat: move plugin dir acquiring to the 'Given' block
try the --activate flag for `wp scaffold plugin` one more time
Merge pull request #451 from wp-cli/behat-refactor

Get rid of "Then it should run without errors" step