Simulate and announce a /wp-admin/ page load

Most of the built-in commands need access to code from
wp-admin/includes/ so it makes sense to load it upfront.

Since we're already doing that, it seems like a good idea to also
announce this fact to plugins, by setting WP_ADMIN to true.

And since we're setting WP_ADMIN to true, we don't need to set up the
global $wp_query instance anymore, because the only admin screen where
it's set is the post list screen (wp-admin/edit.php).

See #385