fix(tests): set trx mtimes via std instead of a new dev-dependency

std::fs::File::set_modified has covered this since Rust 1.75, well under
the crate's 1.91 rust-version, so the filetime dev-dependency buys nothing.
Dropping it also drops the Cargo.toml hunk, which added a second
[dev-dependencies] table and made the manifest a duplicate key once develop
grew its own.

Extend the same treatment to the remaining sleep-ordered mtime test,
merge_test_summary_from_trx_ignores_stale_fallback_file, whose 5 ms sleep
left the stale/fresh distinction to filesystem mtime granularity.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>