provide step or add static cache clearing to cron

[djdevin]

There are some cases where we assume that there is going to be a new page load, or at least a non-statically cached call.

For example:

1. Create a user with Behat (Drupal API)
2. Edit that user, but, through Mink
3. Run cron, which operates on that user.

The "run cron" step in Drupalextension does not clear static caches, so the user is the old user.

There is already a static cache step, but it's not exposed as gherkin yet.

I fixed the issue by calling clearStaticCaches() before running cron.