The random data generator used for tests repeats the same value over and over again

The RandomValues class returns references to its internal buffer as random values. This buffer gets a random value every time a new random value is requested, but since earlier values reference the same internal buffer, they get changed to the same value as well. So even if successive calls return different values each time, the actual list of these values will always consist of a single value repeated multiple times. For example:

|n-th call|returned value|accumulated list expected|accumulated list actual|
|
|-|-|-|-|-|
|1|6C|6C|6C|
|
|2|8F|6C 8F|8F 8F|
|
|3|52|6C 8F 52|52 52 52|
|
|4|B8|6C 8F 52 B8|B8 B8 B8 B8|
|

**Reporter**: [Zoltan Ivanfi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=zi) / @zivanfi
#### PRs and other links:
- [GitHub Pull Request #504](https://github.com/apache/parquet-mr/pull/504)

_{**Note**: *This issue was originally created as [PARQUET-1353](https://issues.apache.org/jira/browse/PARQUET-1353). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Inizia individuando la classe RandomValues e leggi come restituisce il proprio buffer interno. Esamina Pull Request #504 per il lavoro già esistente, quindi verifica che i valori casuali accumulati rimangano distinti tra chiamate successive e che i test pertinenti abbiano esito positivo.

Scritto dal modello di indicizzazione a partire dal testo della issue.