superbench runtime needs to flush log to the result file

[LiweiPeng]

When superbench (version 0.6.0-rc1) runs a test, the test output is saved in some memory. It didn't flush the log to output file. This is hard for users to track the test process, especially for interactive tests, long running tests (>=5minutes).

This feature is especially important because sometime a test may hang. Without output, it is hard to tell whether it is actually hang or not.

Expected: superbench should flush the outputs to the log file immediately.