[tests] fix bbr_5_11_01 random fails (#6589)

Set default BBR registration delay to 2. Defining a smaller BBR jitter
can help all TCs to be more robust.

tests/scripts/thread-cert/config.py

@@ -138,6 +138,7 @@ DEFAULT_CHILD_TIMEOUT = 6
 VIRTUAL_TIME = int(os.getenv('VIRTUAL_TIME', 0))
 PARENT_AGGREGATIOIN_DELAY = 5
 DUA_DAD_DELAY = 5
+DEFAULT_BBR_REGISTRATION_JITTER = 2
 
 LEADER_NOTIFY_SED_BY_CHILD_UPDATE_REQUEST = True
 

tests/scripts/thread-cert/thread_cert.py

@@ -425,6 +425,7 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             # BBRs must use thread version 1.2
             assert params.get('version', '1.2') == '1.2', params
             params['version'] = '1.2'
+            params.setdefault('bbr_registration_jitter', config.DEFAULT_BBR_REGISTRATION_JITTER)
         elif params.get('is_host'):
             # Hosts must not specify thread version
             assert params.get('version', '') == '', params