Add a unit test to verify flush()
Add a unit test to verify flush()

On Windows a test will be skipped since flush() does not work there

Change-Id: Ibda38401aff5f1c50644c6d61f9802217146a4fb
Reviewed-by: Sergey Belyashov <8871b4c2d635fca691211507eadbac3d0bc2695a@gmail.com>
Reviewed-by: Denis Shienkov <6a9c747bd2ba446682324cf340d3b86f30cf4d55@gmail.com>

RoadWorksSoftware/QtSerialPort-OS2,RoadWorksSoftware/QtSerialPort-OS2