[Enhancement] Add proper feature testing library/framework

The CLI provides a useful, but limited testing library for developers, called `dev-container-features-test-lib` to use in test.sh

I propose to add a proper, widely used bash library or framework, so testing can be more reliable.

Suggestions about which framework or library to use, or which possibilities it should have are welcome.

I myself think of 2 options:

1. A library like the one that is used now, but much more reliable and extended. The code is run first, and than the result is checked by assertion commands.
2. A Unit testing framework that can be used for to test the code in isolated parts. It does not run the code first, but checks every function separately.

I think option 1 is fine for now. But if there is demand for Unit testing, we better integrate it right away.

まず `test.sh` における既存の `dev-container-features-test-lib` の使用方法を読み、現在のアサーションと制約を整理します。提案されているライブラリ形式のアプローチとユニットテストのアプローチを比較し、統合する前に framework と必要な機能を定義します。完了の条件は、CLI に信頼性があり、ドキュメント化された feature テストの手段があることです。

索引モデルが issue の本文から書いたものです。