modify test_read_visium_counts to test read_visium function (#2170)
modify test_read_visium_counts to test read_visium function (#2170)

* modify test_read_visium_counts to test read_visium function
* update variable name

Co-authored-by: Isaac Virshup <d9d96763d10860478ad8c9f5058054d5cd289745@gmail.com>
theislab/scanpy,theislab/scanpy