Remove mention of deprecated 'jest-native' in docs

The following pages contain a reference to jest-native which is deprecated:

• versioned_docs/version-6.x/testing.md
• versioned_docs/version-7.x/testing.md

The text says: "We recommend using React Native Testing Library along with jest-native to write your tests."

However, jest-native's github repo says that "This package is deprecated and is no longer actively maintained. We encourage you to migrate to React Native Testing Library, v12.4 or later, which includes modern built-in Jest matchers based on the matchers for this repository."

開啟 versioned_docs/version-6.x/testing.md 和 versioned_docs/version-7.x/testing.md，然後找出提及 jest-native 的建議。更新文件，移除已棄用的參照，並反映 React Native Testing Library 目前的指南。確認這兩個頁面都不再建議使用 jest-native。