[tests] fix key_id_mode handling and add exception handle (#4927)

This commit fixes key_id_mode handling and added exception handling
when key_id_mode is 0.

According to IEEE Std 802.15.4-2015, when Key Id Mode is 0x00:

    Key is determined implicitly from the originator and recipient(s)
    of the frame, as indicated in the frame header.

Our test scripts cannot handle such packets. If we leave key_id_mode =
0 case unhandled, we would get some exceptions that may cause
developers to think that something is wrong. However dropping packets
in this case is a normal behavior.

tests/scripts/thread-cert/mac802154.py

@@ -42,6 +42,15 @@ from net_crypto import (
 )
 
 
+class KeyIdMode0Exception(Exception):
+    """
+    Raised when key id mode of packet is 0.
+    Such packet wouldn't be handled in test scripts,
+    but it's not abnormal behavior.
+    """
+    pass
+
+
 class DeviceDescriptors:
     """Class representing 802.15.4 Device Descriptors."""
 
@@ -377,11 +386,13 @@ class MacFrame:
         key_id_mode = (security_control & 0x18) >> 3
 
         if key_id_mode == 0:
-            key_id = data.read(9)
+            raise KeyIdMode0Exception
         elif key_id_mode == 1:
             key_id = data.read(1)
         elif key_id_mode == 2:
             key_id = data.read(5)
+        elif key_id_mode == 3:
+            key_id = data.read(9)
         else:
             pass