[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.*}

检查当前使用 JUnit 断言的 Java 测试，并确定采用 Google Truth 的范围。检查针对 ValueVector 提议的自定义 subject，包括针对 NullCount 和 Length 的断言。完成标准应包括已达成一致的迁移范围以及可正常工作的 ValueVector 断言，但 issue 未指定具体的文件或测试。

由索引模型根据 Issue 内容生成。