[zh-TW] Translate test-modules.md and doc-tests.md

[johnathan79717]

private -> 私有 is from https://rust-lang.tw/book-tw/ch07-02-defining-modules-to-control-scope-and-privacy.html#:~:text=Asparagus%20%E4%BE%86%E6%89%BE%E5%88%B0%E3%80%82-,%E7%A7%81%E6%9C%89,-vs%20%E5%85%AC%E9%96%8B%EF%BC%9A%E6%A8%A1%E7%B5%84