fix: preserve corpus timeout output

[nightcityblade]

Summary

Preserve compiler stdout and stderr when a corpus check times out.

Motivation

Fixes #593.

Target and compatibility impact

None; diagnostics now match the existing runnable-example timeout behavior.

Validation

• python3 -m unittest tools.test_check_wave_corpus (13 passed)
• python3 -m ruff check tools/check_wave_corpus.py tools/test_check_wave_corpus.py
• git diff --check

Checklist

• Commits include a DCO Signed-off-by line.
• Tests cover new behavior or the PR explains why no test is needed.
• User-facing changes include documentation or diagnostics updates.
• The change preserves the license boundary between the compiler and std/.

[LunaStev]

The implementation is LGTM; just a minor issue with the tests.

But I'll merge it anyway.