Modfied Step: I wait for 5 Seconds to be more robust

Originally, the step was I wait for 5 seconds. This has been modified to I wait until I see "File upload completed!" This means it keeps trying to find the text "File Upload Completed" until it timeouts. 

This is PR is reliant on this being merged in: https://github.com/silverstripe-labs/silverstripe-behat-extension/pull/27