`test_random_big()` fails (or hangs) randomly

[rivy]

Yes, I know it's ironic 😄 ... but it's periodically breaking the build.

• see https://github.com/rivy/rs.coreutils/runs/725535494?check_suite_focus=true
• see https://github.com/rivy/rs.coreutils/runs/724178046?check_suite_focus=true

Ping @nbraud ; any interest or thoughts?

[rivy]

test_factor::test_random_... should be changed to print the used random seed and allow setting that seed for reproducibility and investigation of failed tests.

[nicoonoclaste]

Oops, sorry, I missed this issue entirely, but this should have been fixed by #1529