test_content_source_verbose fails with xz 5.2.8 on guix

When trying to update to diffoscope 267 on guix, an xz test failed:

tests/comparators/test_xz.py::test_content_source_verbose FAILED

=================================== FAILURES ===================================                                                    
_________________________ test_content_source_verbose __________________________                                                    
                                                                                                                                
differences_verbose = [<Difference xz --list -- xz --list []>]                                                                      
                                                              
    @skip_unless_tools_exist("xz")                                                                                                  
    def test_content_source_verbose(differences_verbose):                                                                           
        assert differences_verbose[0].source1 == "xz --list"                                                                        
        assert differences_verbose[0].source2 == "xz --list"                                                                        
>       assert_diff(differences_verbose[0], "text_xz_list")                                                                         
                                                                                                                                
tests/comparators/test_xz.py:89:                                                                                                    
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _                 
                                                                                                                                
difference = <Difference xz --list -- xz --list []>, filename = 'text_xz_list'          
                                                                                                                                
    def assert_diff(difference, filename):                                                                                          
        # Assign seen and expected values to local variables to improve contextual            
        # information in failed tests.                                                                                              
        seen = difference.unified_diff                                                                                              
        expected = get_data(filename)                                                                                               
>       assert seen == expected                                                                                                     
E       AssertionError

Using xz 5.4.5 worked around the problem, but it will be a while before that version can land in guix.

Seems like the test should have a versioned dependency at the very least, somewhere between 5.2.8 and 5.4.5, and/or test data added for older versions.

I did try manually run diffoscope on the output, and I could not catch the difference with my eyeball, although the output was prettified a bit even with the --text output method, so I could not compare directly against the test data provided.

How is the test data normally generated? This is edging into an aside, but could diffoscope have a flag to output in a test-data compatible format?