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:

dask-image/dask_image/imread/__init__.py

Lines 72 to 73 in 329afe7

	ar = dask.array.from_array(filenames, chunks=(nframes,))
	multiple_files = True

[GenevieveBuckley]

Ideally, those filenames should be multi-dimensional data (not just 2D images). We specifically want to prevent regressions like these again: #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.