behat: unify and standardize output testing steps:

* always run replace_variables() and restrict variable names
* add 'not contain' option
first pass at theme tests
always use wp_get_themes()
use wp_get_theme() instead of get_theme_data()
switch_theme() doesn't even accept a second parameter
add tests for `plugin status`
use WP_Theme objects, instead of messing with paths
add theme updating scenario
Merge pull request #348 from wp-cli/fix-338

Make use of WP_Theme