[dts] [PATCH V2] tests/vhost_cbdma: Optimize Case1-2 and Add Case3 to check CBDMA threshold value

[Tu, Lijuan]

> V1:
> 0, Add expected value json file and related function.
> 1, Optimize Case1: Add relaunch vhost performance check.
> 2, Optimize self.send_and_verify function.
> 3, Add New Case3: test_check_threshold_value_with_cbdma.
> 
> V2: Optimize Case3 check point.
> 
> Signed-off-by: JiangYu <yux.jiang at intel.com>

applied