Rename test base class to be more obvious
Rename test base class to be more obvious

It wasn't really obvious that this was a base class and so we spent a
little while trying to work out where on the apiclient being used in the
tests was being imported from (from the subclasses that use it).

Renaming to be more consistent and obvious.

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend