Binary tests check conditions not part of the API contract

`TestBinary#testConstantCopy` validates that when the backing buffer for a `Binary` is changed, the changes can be seen when calling either `Binary#getBytes` or `Binary#getBytesUnsafe`. This is not a part of the API contract for constant binary. The API guarantees the opposite: that if a reused buffer changes then those changes won't affect the value already returned by `getBytes`.

`ByteBufferBackedBinary` caches the value returned by `getBytes`, which is allowed in the API. This causes tests to fail when it is expected that a change to the underlying buffer changes the value of the binary, when the contract for constant buffers is that the underlying buffer will not be changed.

**Reporter**: [Ryan Blue](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=rdblue) / @rdblue

_{**Note**: *This issue was originally created as [PARQUET-414](https://issues.apache.org/jira/browse/PARQUET-414). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

BinaryTest#testConstantCopy から始め、issue で説明されている Binary と ByteBufferBackedBinary の API の動作を調べます。バイナリテストを実行し、その後、定数バッファーのコントラクトに合わせてテストを調整し、バッキングバッファーが変更されたときにキャッシュされた値が変更されることを期待しないようにします。

索引モデルが issue の本文から書いたものです。