[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.*}

Review the Java tests that currently use JUnit assertions and identify the scope for adopting Google Truth. Examine the proposed custom subject for ValueVector, including assertions for NullCount and Length. Done should include an agreed migration scope and working ValueVector assertions, but the issue does not name specific files or tests.

Written by the indexing model from the issue text.