test `process::tests::test_thread_ids` randomly fails

`process::tests::test_thread_ids` in `src/uu/pgrep/src/process.rs` randomly fails.

see https://github.com/uutils/procps/actions/runs/16442491537/job/46466370959

this is not the first time. it also appeared in other PRs, and successed when I rerun it. it seems that it never appears in ci of main branch.