Fix Comprehensive Tests for nl_BE Bank Provider

[AliYmn]

What does this change

This change introduces comprehensive tests for the nl_BE bank provider in Faker. It covers the generation and validation of BBAN, IBAN, and SWIFT codes to ensure correctness and compliance with Belgian banking standards.

What was wrong

The existing implementation for the nl_BE bank provider lacked detailed testing for BBAN, IBAN, and SWIFT code generation. This made it harder to ensure the correctness of generated data, particularly for validation of check digits (e.g., mod-97 algorithm for BBAN and IBAN).

How this fixes it

• Added detailed tests for:
  • BBAN format and check digit validation.
  • IBAN generation, including country code, format validation, and BBAN integration.
  • SWIFT code generation for both 8-character and 11-character formats, with validation for bank, country, and location/branch codes.
• Ensures generated data matches expected Belgian banking standards.
• Validates against external tools and libraries like is_valid_iban.

Fixes #2135

Checklist

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint