I don't know what the old root is, but I know (a) it's set and (b) I don't want it for this test, because I don't want to type `['event_report_row']` in a bunch of places.

`EventReportRowSerializer` is presently only used in association with `EventReportSerializer`, where it generates rootless JSON objects; however, when used in isolation, it uses the default root behavior of an `ActiveModel::Serializer`.

It's probably better to move this test suite to `EventReportSerializer`; that makes it possible to treat `EventReportRowSerializer` as an implementation detail.