[border-router] add test for manual address reachability (#7145)

The purpose of this test case is to verify that BR can manually
configure addresses and comply with the manual address configuration
rules.

tests/scripts/thread-cert/node.py

@@ -3245,6 +3245,7 @@ EOF
 
 
 class OtbrNode(LinuxHost, NodeImpl, OtbrDocker):
+    TUN_DEV = config.THREAD_IFNAME
     is_otbr = True
     is_bbr = True  # OTBR is also BBR
     node_type = 'otbr-docker'
@@ -3257,6 +3258,10 @@ class OtbrNode(LinuxHost, NodeImpl, OtbrDocker):
         self.set_log_level(5)
         super().start()
 
+    def add_ipaddr(self, addr):
+        cmd = f'ip -6 addr add {addr}/64 dev {self.TUN_DEV}'
+        self.bash(cmd)
+
 
 class HostNode(LinuxHost, OtbrDocker):
     is_host = True