Improve the delete collection test

Ensure that the data set was *actually* created successfully before
trying to delete it. This was to help us track down why collections
don't seem to be deleted successfully from Stagecraft:

https://www.pivotaltracker.com/story/show/70503022

[#70503022]
Merge pull request #277 from alphagov/improve-delete-test

Improve the delete collection test