Backend: Re-write "Convey" tests to use testify

[fridgepoet]

Over time, Grafana's backend testing style has moved from using Convey to testify. For example grafana/grafana#26724
Current to the style guide https://github.com/grafana/grafana/blob/main/contribute/backend/style-guide.md#testing

This remains to be done in this OpenSearch datasource.

It may be helpful to reference the Elasticsearch datasource in the main Grafana repo from which this repo originated. Its tests are already written using testify. https://github.com/grafana/grafana/tree/main/pkg/tsdb/elasticsearch

Note: Marking this as a "good first issue" for anyone who would like to contribute any part of this -- the overall work may be significant, but some tests may be straightforward to re-write.