[Java] Consider starting to use Google Truth Fluent Assertions library

This can offer more readable asserts than the limited JUnit assertions.

we might want to add see if we can add a custom subject, that allows for easily asserting things about ValueVector's (e.g. NullCount, Length, etc).

**Reporter**: [Micah Kornfield](https://issues.apache.org/jira/browse/ARROW-6931) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-6931](https://issues.apache.org/jira/browse/ARROW-6931). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

檢查目前使用 JUnit 斷言的 Java 測試，並確定採用 Google Truth 的範圍。檢查針對 ValueVector 提議的自訂 subject，包括針對 NullCount 和 Length 的斷言。完成標準應包括已達成共識的遷移範圍以及可正常運作的 ValueVector 斷言，但 issue 未指定具體的檔案或測試。

由索引模型根據 Issue 內容生成。