fix(tests): make dotnet TRX tests deterministic via explicit mtimes

[AngelVRodC]

Fixes #4037

Summary

• Replace wall-clock-dependent fixture mtimes in three flaky dotnet tests with explicit File::set_modified timestamps (old = base − 1 day, new = base + 1 hour), deriving every since cutoff from the same base so results are independent of filesystem mtime granularity (HFS+/FAT 1–2s flooring).
• Remove all thread::sleep synchronization from these tests; add a tiny #[cfg(test)] write_trx_with_mtime helper in src/cmds/dotnet/mod.rs (inline module — automod::dir! would declare a new file as a production module).
• Fixes the reported flake in test_parse_trx_files_in_dir_since_ignores_older_files plus two sibling latent flakes found during investigation (test_find_recent_trx_in_dir_picks_newest_trx, test_merge_test_summary_from_trx_ignores_stale_fallback_file).

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test — 0 warnings, 3781 passed / 0 failed
• Repeated the dotnet TRX test suites 20× consecutively — 0 flakes, and the suites no longer sleep (each run finishes in ~0.02s)
• Test-only diff: 3 files in src/cmds/dotnet/, +37/−16; no production code, no new dependencies

[rtk-wshm-sync-bot]

wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟢 Risk	low

Summary

Fixes flaky dotnet TRX tests by replacing wall-clock-dependent mtimes and thread::sleep synchronization with explicit File::set_modified timestamps derived from a shared base time. A new #[cfg(test)] helper write_trx_with_mtime is added to pin fixture mtimes precisely, eliminating filesystem mtime granularity issues.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #4037

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Analyzed automatically by wshm · This is an automated analysis, not a human review.

[KuSh]

Thanks for the fix and for finding the two sibling flakes. Issue #4037 was closed by #4048 (merged 2026-09-17), which covers the same three tests: old fixtures are pinned ten seconds before the cutoff and new ones ten seconds after through an explicit set_modified, and the sleeps are gone, so the dotnet TRX suites no longer depend on filesystem mtime granularity. The shared write_trx_with_mtime helper here is the one thing develop does not have, and with a per module set_mtime helper already in place that did not look like a reason to keep this open. Please comment or reopen if you think a part of this was missed.