[tests] update meshcop service test case (#6962)

The spec asks border router to publish meshcop service even when
thread interface is inactive. This commit adds test case to verify the
feature.

This commit also introduces boot_delay to add delays between booting
nodes. In this way we can let border routers not starting at the same
time, so that there will be fewer renaming conflicts. This can make
the test more stable.

tests/scripts/thread-cert/thread_cert.py

@@ -162,6 +162,8 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
                 version=params['version'],
                 is_bbr=params['is_bbr'],
             )
+            if 'boot_delay' in params:
+                self.simulator.go(params['boot_delay'])
 
             self.nodes[i] = node