Non-zero exit code caused by lots of `\showbox`

[muzimuzhi]

@moewew reported, in https://github.com/latex3/l3build/issues/327#issuecomment-1865104579, that

Hmm, I'm using lots of \showbox which I think halts the TeX run and probably lets it end with a non-zero exit code. Is there an alternative for that?

@u-fischer then provided a \showstream workaround and @FrankMittelbach suggested providing a new command \SHOWBOX that "encaps that workflow".

\documentclass{article}
\input{regression-test}
\begin{document}
\START
\newwrite\boxout
\immediate\openout\boxout{box.txt}
\showstream\boxout
\setbox0\hbox{duck}
\showbox0
\setbox0\hbox{bär}
\showbox0
\immediate\closeout\boxout
\SHOWFILE{box.txt}
xxx
\end{document}

I'd like to check that the reported problem is reproducible first.