badserverext: explicitly flush each log write

No idea why, but without this, there was no content in the error logs that are
`cat`d in the test on Windows.

tests/badserverext.py

@@ -111,6 +111,7 b' class fileobjectproxy(object):'
 
         object.__getattribute__(self, '_logfp').write(msg)
         object.__getattribute__(self, '_logfp').write('\n')
+        object.__getattribute__(self, '_logfp').flush()
 
     def read(self, size=-1):
         remaining = object.__getattribute__(self, '_closeafterrecvbytes')