The only difference is that the json:{} filename of the image looks
different.  We actually do not care about that filename in this test, we
are only interested in (1) that there is a json:{} filename, and (2)
whether the backing filename can be constructed.

So just filter out the json:{} data, thus making this test pass both
with and without data_file.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-19-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>