test: drop environment assumptions from fixtures

[KiryuuLight]

Two tests depended on the environment they were written in.

• ui/log.rs wrapping fixtures hardcoded /home/light. The path is never read — the tests only care about string length — and it shipped in the 0.1.1 crate on crates.io.
• paths.rs::user_placeholder_matches_the_environment called env::var("USER").unwrap(), which panics wherever USER isn't set: build sandboxes, minimal containers, some su/cron setups. The test also disagreed with the implementation, which falls back to LOGNAME and then leaves {user} literal. It now checks the same sources the implementation does and asserts the literal-passthrough case.

Verified with cargo test both normally and under env -u USER -u LOGNAME — 69 passing either way.