[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/thread_cert.py

@@ -38,7 +38,7 @@ import sys
 import time
 import traceback
 import unittest
-from typing import Optional, Callable
+from typing import Optional, Callable, Union, Any
 
 import config
 import debug
@@ -101,6 +101,7 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
     TOPOLOGY = None
     CASE_WIRESHARK_PREFS = None
     SUPPORT_THREAD_1_1 = True
+    PACKET_VERIFICATION = config.PACKET_VERIFICATION_DEFAULT
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
@@ -108,9 +109,16 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
         logging.basicConfig(level=logging.DEBUG, format='%(asctime)s - %(levelname)s - %(message)s')
 
         self._start_time = None
-        self._do_packet_verification = PACKET_VERIFICATION and hasattr(self, 'verify')
+        self._do_packet_verification = PACKET_VERIFICATION and hasattr(self, 'verify') \
+                                       and self.PACKET_VERIFICATION == PACKET_VERIFICATION
+
+    def skipTest(self, reason: Any) -> None:
+        self._testSkipped = True
+        super(TestCase, self).skipTest(reason)
 
     def setUp(self):
+        self._testSkipped = False
+
         if ENV_THREAD_VERSION == '1.1' and not self.SUPPORT_THREAD_1_1:
             self.skipTest('Thread 1.1 not supported.')
 
@@ -290,7 +298,8 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             self._test_info['pcap'] = pcap_filename
 
             test_info_path = self._output_test_info()
-            self._verify_packets(test_info_path)
+            if not self._testSkipped:
+                self._verify_packets(test_info_path)
 
     def flush_all(self):
         """Flush away all captured messages of all nodes.
@@ -558,8 +567,9 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             if timeout <= 0:
                 raise RuntimeError(f'wait failed after {timeout} seconds')
 
-    def wait_node_state(self, nodeid: int, state: str, timeout: int):
-        self.wait_until(lambda: self.nodes[nodeid].get_state() == state, timeout)
+    def wait_node_state(self, node: Union[int, Node], state: str, timeout: int):
+        node = self.nodes[node] if isinstance(node, int) else node
+        self.wait_until(lambda: node.get_state() == state, timeout)
 
     def wait_route_established(self, node1: int, node2: int, timeout=10):
         node2_addr = self.nodes[node2].get_ip6_address(config.ADDRESS_TYPE.RLOC)