twister: Fix DeviceHandler serial leftover cleanup

Fix how DeviceHandler class does 'serial leftover' cleanup
with loss of test output observed on fast console connections,
e.g. over telnet: either 1 sec. of output might be missed,
or up to 1000 bytes/chars, whatever happens first.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>