`kedro-datasets`: Fix tests after replacing `__str__ ` with `__repr__` for `AbstractDataset`

## Description
After this change https://github.com/kedro-org/kedro/pull/4929, tests for datasets should be updated correspondingly.

## Context
https://github.com/kedro-org/kedro-plugins/actions/runs/16047020178/job/45280666929?pr=1095
