New test needed: imread given multiple filenames

[GenevieveBuckley]

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

[GenevieveBuckley]

Ideally, those filenames should be multi-dimensional data (not just 2D images). We specifically want to prevent regressions like these again: https://github.com/dask/dask-image/issues/220

In a pinch, we could construct dummy test files to read in - the pytest tmpdir fixture should handle cleanup for us if we do that.