Revert "no need to flush stderr in my tests"

It is actually necessary, for now.

This reverts commit 8fdd1eddf9.

Conflicts:
	borg/helpers.py

borg/helpers.py

@@ -197,6 +197,7 @@ class Statistics:
         else:
             msg = ' ' * columns
         print(msg, file=stream or sys.stderr, end="\r")
+        (stream or sys.stderr).flush()
 
 
 def get_keys_dir():