docs/zh_CN: Add translation of rust/testing.rst

docs/zh_CN: Add translation of rust/testing.rst

Complete the translation of rust/testing.rst and add the testing TOC entry
to rust/index.rst.

Add the translation based on commit a3b2347343e0
("Documentation: rust: testing: add docs on the new KUnit `#[test]` tests").

Signed-off-by: Ben Guo <benx.guo@gmail.com>
Reviewed-by: Yanteng Si <siyanteng@cqsoftware.com.cm>
Signed-off-by: Alex Shi <alexs@kernel.org>