du: already passes GNU test in spirit, adjust test

[BenWiederhake]

This PR adapts a GNU test to accept our phrasing of the error message.

I was looking at low-hanging fruit on the table, and dived straight into du/move-dir-while-traversing.

This GNU test exists to ward against a regression: an old GNU coreutils version used to contain a bug that triggered an assertion failure. The test's intention is to make sure that du fails not due to an assertion error (thus not producing any information, or corrupt output), but rather due to the correct reason (files vanishing). In particular, we simply emit a slightly different error message than GNU, and that's why we technically didn't pass that test.

However, since we pass the spirit of the test, I think it is appropriate to adapt the test to check for our phrasing of the error message.

While I was at it, I refreshed one of the other quilt patches, since the line numbers changed. I made sure to use GNU coreutils at git tag v9.7. I ignored two other diffs introduced by quilt refresh, since I'm not convinced they are correct.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/du/move-dir-while-traversing is no longer failing!

[BenWiederhake]

CI failure test_tail::test_follow_when_files_are_pointing_to_same_relative_file_and_file_stays_same_size seems to be a flake.

[sylvestre]

excellent!