Refactor TestApproveAgreement and TestUnapproveAgreement
Refactor TestApproveAgreement and TestUnapproveAgreement

As the method return value is freshly patched in the setup
method, we no longer need the separate property in each class.

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend