Add test suite based on RFC 4648 test vectors and validation rules:
- Add section 10 test vectors for valid encoding/decoding
- Add test cases for invalid character handling (section 3.2)
- Add test cases for padding validation (section 4)
- Add test cases for length requirements (section 6)
The test vectors verify that:
- Empty string encodes/decodes correctly
- Standard cases like "foo" -> "MZXW6===" work
- Invalid characters are rejected
- Missing or malformed padding is detected
- Invalid lengths are caught
Zoltán Kéri
7672ac945a