Simplify tests for exception handling in CsvPreviewControllerTest
Simplify tests for exception handling in CsvPreviewControllerTest

Having extracted the CsvFileFromPublicHost.csv_preview method in an
earlier commit, we can now simplify the test so we just stub this method
and return an instance of CsvPreview; there's no longer any need to stub
anything on the CsvPreview class.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall