[trel] small updates and adding trel test (#7205)

- Adds platform API `trelDnssdInitialize` to initialize TREL
  DNS-SD module.
- Adds `test_trel_connectivity.py` test.
- Fixed `Border Router` TREL test to really use TREL.

tests/scripts/thread-cert/config.py

@@ -147,6 +147,10 @@ LEADER_NOTIFY_SED_BY_CHILD_UPDATE_REQUEST = True
 THREAD_VERSION_1_1 = 2
 THREAD_VERSION_1_2 = 3
 
+PACKET_VERIFICATION_NONE = 0
+PACKET_VERIFICATION_DEFAULT = 1
+PACKET_VERIFICATION_TREL = 2
+
 
 def create_default_network_data_prefix_sub_tlvs_factories():
     return {