Added test cases for dynamodb_tables_pitr_enabled

[Ninad-Lunge-Comprinno]

Context

This PR adds a test suite for the dynamodb_tables_pitr_enabled check to validate that the check behaves correctly in different scenarios — such as when no tables exist, when PITR is enabled/disabled, and when exceptions occur.

Description

• Added unit tests using botocore.stub.Stubber to simulate AWS DynamoDB responses.

• Covered scenarios:

  • No DynamoDB tables present.
  • All tables have PITR enabled.
  • Some tables have PITR disabled.
  • Exception occurs while fetching PITR status.
  • Exception occurs while listing tables.

• Introduced a DummySession to mock boto3 sessions for injection.

No new dependencies were introduced. The test cases help ensure the check's robustness across failure modes and typical AWS configurations.

Checklist

• Added new checks? If yes, reviewed necessary permissions – N/A (tests only)
• Code covered by tests (if not, explain why)
• Documentation follows [Google Python Style Guide](https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings)
• Considered if backporting is needed – Not needed

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.