compiletest: never truncate stderr if we'll use it as json

[durin42]

I started observing some strange failures in larger ui tests that were
caused by environmental conditions (probably paths?) pushing the stderr
size over the truncation limit, causing the tests to fail because the ui
output wasn't valid json due to the truncation marker. Since that seems
like a circumstance we never want, just never truncate output that we'll
interpret as json output.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[Mark-Simulacrum]

#96551 is probably the approach we're going to pursue here - the truncation is not itself useful, but rather a mechanism to avoid e.g. oom or other hangs in CI. It also helps keep the amount of output down.

[Mark-Simulacrum]

Going to close in favor of that PR, but feel free to comment there if you disagree or have thoughts.

[durin42]

That PR looks like a better overall path. I look forward to it landing! Thanks!