[github-actions] make test_set_mliid more reliable (#7072)

This commit uses a longer duration (10s) to make the test more
reliable.

The root cause why LEADER does not become Leader within 4s is still
unknown and to be investigated.

tests/scripts/thread-cert/test_set_mliid.py

@@ -50,7 +50,7 @@ class Test_SetMlIid(thread_cert.TestCase):
         self.nodes[LEADER].set_mliid('1122334455667788')
 
         self.nodes[LEADER].start()
-        self.simulator.go(4)
+        self.simulator.go(10)
         self.assertEqual(self.nodes[LEADER].get_state(), 'leader')
 
         # Ensure set ML-IID was effective.