[test] support multiple backbone interfaces in otbr docker test (#10550)

In previous otbr docker tests, when creating docker containers, all the
containers(otbr nodes) are connected to the same docker network bridge `backbone0`
(when the env PORT_OFFSET is not set or set to 0), this means all the
otbr nodes are connected to the same infrastructures.

This commit adds support to enable user to config otbr instance
infrastructures seperately in `TOPOLOGY` when defining test cases, this
provides flexibility to run multi-ail related test cases.

The format to define backbone interface per node is:

```
class NewTestCase(thread_cert.TestCase):
    ...
    BR = 1
    ...
    TOPOLOGY = {
        BR: {
            ...
	    'is_otbr': True,
	    'backbone': <backbone-name>,
	    ...
	}
	...
    }
    ...
```

`'backbone': <backbone-name>` is optional, when it's not given when
defining a otbr node, the backbone is default as
`BACKBONE_DOCKER_NETWORK_NAME`. The `<backbone-name>` is suggested to be
defined as `backbone[0-9]` to make it more easy to read and understand.

This commit also adds test_multi_ail.py as an example test case to use
this new method, this test case checks the two otbr nodes are connected
to the different infra and are in the same Thread mesh network.

tests/scripts/thread-cert/thread_cert.py

@@ -162,14 +162,13 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             else:
                 nodeclass = Node
 
-            node = nodeclass(
-                i,
-                is_mtd=params['is_mtd'],
-                simulator=self.simulator,
-                name=params.get('name'),
-                version=params['version'],
-                is_bbr=params['is_bbr'],
-            )
+            node = nodeclass(i,
+                             is_mtd=params['is_mtd'],
+                             simulator=self.simulator,
+                             name=params.get('name'),
+                             version=params['version'],
+                             is_bbr=params['is_bbr'],
+                             backbone_network=params['backbone_network'])
             if 'boot_delay' in params:
                 self.simulator.go(params['boot_delay'])
 
@@ -501,6 +500,9 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             assert params.get('version', '') == '', params
             params['version'] = ''
 
+        # set default backbone network for bbr/otbr/host if not specified
+        params.setdefault('backbone_network', config.BACKBONE_DOCKER_NETWORK_NAME)
+
         # use 1.3 node for 1.2 tests
         if params.get('version') == '1.2':
             params['version'] = '1.3'
@@ -524,10 +526,30 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
         return False
 
     def _prepare_backbone_network(self):
-        network_name = config.BACKBONE_DOCKER_NETWORK_NAME
-        self.assure_run_ok(
-            f'docker network create --driver bridge --ipv6 --subnet {config.BACKBONE_PREFIX} -o "com.docker.network.bridge.name"="{network_name}" {network_name} || true',
-            shell=True)
+        """
+        Prepares one or multiple backbone network(s).
+
+        This method creates the backbone network based on the `backbone` values defined in the TOPOLOGY in each test.
+        If no backbone values are defined, it sets the default backbone network as BACKBONE_DOCKER_NETWORK_NAME
+        from the config module.
+        """
+        # Use backbone_set to store all the backbone values in TOPOLOGY
+        backbone_set = set()
+        for node in self.TOPOLOGY:
+            backbone = self.TOPOLOGY[node].get('backbone_network')
+            if backbone:
+                backbone_set.add(backbone)
+
+        # Set default backbone network if backbone_set is empty
+        if not backbone_set:
+            backbone_set.add(config.BACKBONE_DOCKER_NETWORK_NAME)
+
+        # Iterate over the backbone_set and create backbone network(s)
+        for offset, backbone in enumerate(backbone_set, start=PORT_OFFSET):
+            backbone_prefix = f'{config.BACKBONE_IPV6_ADDR_START_BASE + offset:04x}::/64'
+            self.assure_run_ok(
+                f'docker network create --driver bridge --ipv6 --subnet {backbone_prefix} -o "com.docker.network.bridge.name"="{backbone}" {backbone} || true',
+                shell=True)
 
     def _remove_backbone_network(self):
         network_name = config.BACKBONE_DOCKER_NETWORK_NAME