twister: Fix serial open for UART devices that do not immediately appear

Testing on linux, --flash-before argument did not work to wait for jlink
upload to complete. Instead Serial port opening throws file not found. The
fix ensures that the code waits for the serial port to appear before
attempting to connect. Tested on a proprietary platform with virtual UART
through usb.

Signed-off-by: Eemil Visakorpi <eemil@doublepoint.com>

scripts/pylib/twister/twisterlib/handlers.py

@@ -27,6 +27,7 @@ from pathlib import Path
 from queue import Empty, Queue
 
 import psutil
+from serial.tools import list_ports
 from twisterlib.environment import ZEPHYR_BASE, strip_ansi_sequences
 from twisterlib.error import TwisterException
 from twisterlib.hardwaremap import DUT
@@ -867,6 +868,10 @@ class DeviceHandler(Handler):
                     # Return to normal boot
                     ser.rts = False
                 else:
+                    while ser.port not in (p.name for p in list_ports.comports()):
+                        time.sleep(0.1)
+                        if time.time() - start_time > flash_timeout:
+                            break
                     ser.open()
 
             except serial.SerialException as e: