twisterlib: drain the serial leftover before new test
twisterlib: drain the serial leftover before new test

Some tests can cause serial leftover logs buffered on board.
A simple ser.flush() is not enough. So add explicit readline()
to drain such logs which prepares a clean serial context for
the case that follows.

Signed-off-by: Ming Shao <6bd49e8bc84587c4ae9697e13665e9c193a3e343@intel.com>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr