Fix return-code when specified to be 0

[Lnk2past]

When the return code is explicitly specified to be 0, the return-code check is skipped

The problem is at contest/TestCase.py#L142:

            if self.return_code and int(self.return_code) != proc.returncode:

should be:

            if self.return_code is not None and int(self.return_code) != proc.returncode: