Fix use of fflush and enhance filebuf tests
Flamefire
| Files | Coverage |
|---|---|
| utf | 100.00% |
| args.hpp | 95.83% |
| convert.hpp | 100.00% |
| filebuf.hpp | 94.19% |
| filesystem.hpp | 100.00% |
| fstream.hpp | 100.00% |
| stackstring.hpp | 100.00% |
| stat.hpp | 100.00% |
| utf8_codecvt.hpp | 100.00% |
| Folder Totals (9 files) | 98.07% |
| Project Totals (33 files) | 99.37% |