twisterlib: re-add the logic to drain the serial leftover

This logic was added before but seems to be missed during the
twister refactoring.

Some tests can cause serial leftover logs buffered somewhere.
Such leftover can interfere with the next test case because the
serial log monitoring thread is started before a board is flashed.
And the monitoring thread can be fooled by such leftover logs and
make incorrect judgement of the test result.

A simple ser.flush() is not enough to eliminate such leftovers.
So add explicit readline() to drain such logs which ensures a clean
serial context for the case that follows.

An example from reel board captured with this patch:

leftover log of previous test: b'.287 seconds\r\n'
leftover log of previous test: b' - SKIP - [...test_coredump_backend]...
leftover log of previous test: b'\r\n'
leftover log of previous test: b'------ TESTSUITE SUMMARY END ------\r\n'
leftover log of previous test: b'\r\n'
leftover log of previous test: b'=====================================...
leftover log of previous test: b'RunID: 4e93757ad...53dcab9f0f5c6\r\n'
leftover log of previous test: b'PROJECT EXECUTION SUCCESSFUL\r\n

Signed-off-by: Ming Shao <ming.shao@intel.com>