test_truncate_applies_before_complement_with_class intermittent failure

---- test_tr::test_truncate_applies_before_complement_with_class stdout ----
bin: "D:\\a\\coreutils\\coreutils\\target\\debug\\coreutils.exe"
run: D:\a\coreutils\coreutils\target\debug\coreutils.exe tr -ct [:digit:] X

thread 'test_tr::test_truncate_applies_before_complement_with_class' (7868) panicked at tests\uutests\src\lib\util.rs:2846:18:
called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "failed to write to stdin of child: The pipe is being closed. (os error 232)" }
stack backtrace:
   0: std::panicking::panic_handler
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\std\src\panicking.rs:689
   1: core::panicking::panic_fmt
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\core\src\panicking.rs:80
   2: core::result::unwrap_failed
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\core\src\result.rs:1867
   3: enum2$<core::result::Result<tuple$<>,std::io::error::Error> >::unwrap
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\core\src\result.rs:1233
   4: uutests::util::UChild::join
             at .\tests\uutests\src\lib\util.rs:2846
   5: uutests::util::UChild::wait_with_output
             at .\tests\uutests\src\lib\util.rs:2562
   6: uutests::util::UChild::wait
             at .\tests\uutests\src\lib\util.rs:2537
   7: uutests::util::UCommand::run
             at .\tests\uutests\src\lib\util.rs:2048
   8: uutests::util::UCommand::fails
             at .\tests\uutests\src\lib\util.rs:2072
   9: tests::test_tr::test_truncate_applies_before_complement_with_class
             at .\tests\by-util\test_tr.rs:389
  10: tests::test_tr::test_truncate_applies_before_complement_with_class::closure$0
             at .\tests\by-util\test_tr.rs:385
  11: core::ops::function::FnOnce::call_once<tests::test_tr::test_truncate_applies_before_complement_with_class::closure_env$0,tuple$<> >
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\core\src\ops\function.rs:250
  12: core::ops::function::FnOnce::call_once
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library\core\src\ops\function.rs:250
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

https://github.com/uutils/coreutils/actions/runs/25015365304/job/73261655322?pr=12043

Start with tests/by-util/test_tr.rs around test_truncate_applies_before_complement_with_class at lines 385-389, then inspect tests/uutests/src/lib/util.rs around UChild::join and wait_with_output. Reproduce the intermittent Windows failure from the linked Actions run and determine why the stdin pipe closes; done means the test runs reliably without the reported panic.