use subprocess.check_output to call jzarr test script

[grlee77]

I verified that the following is actually running the script and will fail if an invalid ds_name is given.

For example, if I call it with (nonexistent) ds_name = 'raw2' I get (I replaced a long local path with <DATA_PATH> for brevity):

CalledProcessError: Command 'generate_data/jzarr/generate_data.sh -verify '<DATA_PATH>/jzarr_flat.zr' raw2' returned non-zero exit status 2.

[grlee77]

If you wanted it to print the stdout each time, I would modify slightly to:

    stdout = subprocess.check_output(cmd, shell=True, universal_newlines=True)
    print(stdout)

but pytest will suppress this output by default when running the tests. (calling pytest with -s in the command line arguments would show it, but this is dozens of lines each time, so it gets pretty cluttered).