scripts: pylib: twister: fix missing newlines at handler.log

Keep new lines at handler.log.

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>

Author: nordic-piks

scripts/pylib/twister/twisterlib/handlers.py

@@ -444,7 +444,7 @@ def monitor_serial(self, ser, halt_event, harness):
             # is available yet.
             if serial_line:
                 # can be more lines in serial_line so split them before handling
-                for sl in serial_line.decode('utf-8', 'ignore').splitlines():
+                for sl in serial_line.decode('utf-8', 'ignore').splitlines(keepends=True):
                     log_out_fp.write(strip_ansi_sequences(sl).encode('utf-8'))
                     log_out_fp.flush()
                     if sl := sl.strip():