test: remove `unwrap()` in test helpers and print explicit error line context (#1173)

Replace `unwrap()` based test helper failures with explicit error handling so failures
show actionable diagnostics instead of generic panic messages.

- Extract shared test error formatting into src/test_helpers.rs
- Print stage-specific failures (parse/minify/print)
- Include caller file:line with #[track_caller]
- Render CSS error line/column with nearby source lines and caret indicator