[#49910675] Fix mirrorer tests
[#49910675] Fix mirrorer tests

Method `add_error` was renamed to `add_log_warning` in 4a4094b.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet