New test needed: imread given multiple filenames

We need a test for `imread` when given multiple input image filenames.

The test coverage results show that these lines are uncovered:

https://github.com/dask/dask-image/blob/329afe7499a3c699fb503643b81e91c3d5d4bc81/dask_image/imread/__init__.py#L72-L73

Start by reading dask_image/imread/__init__.py at lines 72-73 to understand the uncovered multiple-filename path. Find the existing imread tests and run that test group before adding coverage for multiple input image filenames; done means the scenario is exercised and the tests pass.

Written by the indexing model from the issue text.