It did not matter before, but now that _make_test_img understands -o, we
should use it properly here.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-11-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>