Tests for copy data migration stuff, add client to requirements

These tests expect a method `migrate_data_set` which will take an old data set config, override specified parts of it and create a new data set with this specification (or update an existing one that matches this identifiers of the exising one). Then it will load the old data sets data using the backdrop client, convert the data to a new format defined by `data_mapping` and post it to the backdrop data-set for the newly created data set config.
Add functions to migrate data sets which meet the specification fo the test

This could be refactored into an object as in the existing mass update script. It should perhaps take the whole mapping itself rather than receiving the three parts sensibly. We should also stop using the deprecrated find by name (we should use group and type)
Migration for the two mislabeled transactions by channel data sets

We are waiting to hear from cliff whether we also need to change the values for weekly claims to clerical_received being paper and so on. As monthly claims is not used we will not migrate this into the new carers allowance transaction by channel data set. There may be a requirement for other data sets to be combined in future however so we should determine if a script can add data to an existing data set already or if we will need to amend it. Adding to an existing data set with this script will require amending auto id so values are not overwritten.