testing/fluent-bit: upgrade to 3.1.9, fix build with gcc 14, enable tests
https://github.com/fluent/fluent-bit/releases/tag/v3.1.9
https://github.com/fluent/fluent-bit/releases
- Fix a rebuild error with gcc 14 initially found in 3.0.7 but also in 3.1.9
- Enable some tests