[tests] initialize mesh-local prefix instead of extended pan id (#7668)

diff --git a/tests/scripts/thread-cert/config.py b/tests/scripts/thread-cert/config.py
index 22958f6..62f36bb 100755
--- a/tests/scripts/thread-cert/config.py
+++ b/tests/scripts/thread-cert/config.py
@@ -45,7 +45,6 @@
 from tlvs_parsing import SubTlvsFactory
 
 # This extended address will generate the MESH_LOCAL_PREFIX
-EXTENDED_PANID = '000db80000000000'
 MESH_LOCAL_PREFIX = 'fd00:db8::/64'
 MESH_LOCAL_PREFIX_REGEX_PATTERN = '^fd00:0?db8:0{0,4}:0{0,4}'
 ROUTING_LOCATOR = '64/:0:ff:fe00:/16'
diff --git a/tests/scripts/thread-cert/node.py b/tests/scripts/thread-cert/node.py
index 5d256e5..50b3694 100755
--- a/tests/scripts/thread-cert/node.py
+++ b/tests/scripts/thread-cert/node.py
@@ -614,7 +614,7 @@
 
         super().__init__(nodeid, **kwargs)
 
-        self.set_extpanid(config.EXTENDED_PANID)
+        self.set_mesh_local_prefix(config.MESH_LOCAL_PREFIX)
         self.set_addr64('%016x' % (thread_cert.EXTENDED_ADDRESS_BASE + nodeid))
 
     def _expect(self, pattern, timeout=-1, *args, **kwargs):
@@ -1417,6 +1417,14 @@
         self.send_command('extpanid')
         return self._expect_result('[0-9a-fA-F]{16}')
 
+    def get_mesh_local_prefix(self):
+        self.send_command('prefix meshlocal')
+        return self._expect_command_output()[0]
+
+    def set_mesh_local_prefix(self, mesh_local_prefix):
+        self.send_command('prefix meshlocal %s' % mesh_local_prefix)
+        self._expect_done()
+
     def get_joiner_id(self):
         self.send_command('joiner id')
         return self._expect_result('[0-9a-fA-F]{16}')