Test inline adding of file(s) to block via Behat

[raissanorth]

With #13 merged and Banner Blocks being inline editable we should check if files can be added inline.

The attempt was made to do so via the Behat tests added to this module, but there are some remaining issues on providing test files to the test database in order to test adding them via the UI in a Behat tests.

silverstripe-asset-admin may be a good reference point for such a task.