Add tests/multithread.py, can replicate a few failures:

- depthai.XLinkWriteError: Couldn't write data to stream: '__bootloader' (X_LINK_ERROR)
- RuntimeError: Error trying to connect to device
- ...ping was missed, closing the device connection

Tested on PoE, 5+ devices connected.
Fix will follow in next commit

tests/multithread.py

@@ -0,0 +1,46 @@
+# TODO: increase defaults in depthai! Below is a workaround for this issue on some setups:
+#     Failed to find device after booting, error message: X_LINK_DEVICE_NOT_FOUND
+# If this error is still observed, might be a FW crash on initialization
+if 1:
+    import os
+    os.environ["DEPTHAI_WATCHDOG_INITIAL_DELAY"] = "40000"
+    os.environ["DEPTHAI_BOOTUP_TIMEOUT"] = "40000"
+
+import depthai as dai
+import threading
+import traceback
+import time
+import os
+
+def worker(devinfo, id):
+    print(f'[{id}] opening bootloader...')
+    try:
+        with dai.DeviceBootloader(devinfo) as bl:
+            print(f'[{id}] bootloader version: {bl.getVersion()}')
+        with dai.Device(devinfo) as dev:
+            print(f'[{id}] device started, name: {dev.getDeviceName()}')
+            time.sleep(6)
+    # Exceptions aren't propagated, so simply exit the app with an error
+    except Exception as e:
+        print(f'[{id}] FAILED: {e}')
+        traceback.print_exc()
+        os._exit(-1)
+
+device_infos = dai.Device.getAllAvailableDevices()
+n = len(device_infos)
+if n <= 0:
+    print('No devices found!')
+    os._exit(-1)
+print(f'Found {n} devices, handling each in a thread...')
+threads = []
+for devinfo in device_infos:
+    id = devinfo.name  # IP address for PoE
+    t = threading.Thread(target=worker, args=(devinfo, id, ))
+    t.name += f'-{id}'  # Thread name useful for debugging
+    t.start()
+    threads.append(t)
+
+for t in threads:
+    t.join()
+
+print('Multi-threading test succeeded!')