[test] fix potential backbone name conflict when running cert suite (#10596)

### Background

https://github.com/openthread/openthread/pull/10550 introduced a new
way to support multiple backbone nework in otbr docker test. Though it
works good while running a single test, a bug exists when running
cert-suite, which runs a batch of tests in parallel.

cert-suite allocates the name of the backbone interfaces dynamically
by setting env PORT_OFFSET for each test, so there is potentially
conflict exists if we hard code the `backbone_network` name in
TOPOLOGY. This PR is targeting to fix this potential naming conflict.

### Fix

We fix it by assigning a number for `backbone_network_id` in each BR
definition in TOPOLOGY, instead of setting a fixed `backbone network`
name. The final backbone network name is decided by both `PORT_OFFSET`
env and the number of `backbone_network` (in
`backbone{PORT_OFFSET}.{backbone_network}` format)

For example, if `PORT_OFFSET` is 0 and `backbone_network_id` is 1,
then backbone network name will be `backbone0.1`. For the tests that
only use one backbone network and the `backbone_network_id` is not
given, the backbone network name is by default
`backbone{PORT_OFFSET}.0`.

### New test case format
```
class NewTestCase(thread_cert.TestCase):
    ...
    BR = 1
    ...
    TOPOLOGY = {
        BR: {
            ...
            'is_otbr': True,
            'backbone_network_id': <backbone-id>,
            ...
        }
        ...
    }
    ...
```

`<backbone-id>` is any integer from 0, for each BR inside a single
test, if `<backbone-id>` is different, the BR use different backbone
interfaces; the same `<backbone-id>` inside a single test case means
the same backbone network interface.

`'backbone_network_id': <backbone-id>` is optional for single backbone
test cases, when it's not given while defining a otbr node, the
backbone is default as `backbone{PORT_OFFSET}.0`.

For developers, if you are defining a new test which has multiple
backbone interfaces, please ensure `backbone_network_id` is explicitly
defined in each BR, otherwize an error is reported.

tests/scripts/thread-cert/node.py

@@ -66,6 +66,8 @@ class OtbrDocker:
 
     def __init__(self, nodeid: int, backbone_network: str, **kwargs):
         self.verbose = int(float(os.getenv('VERBOSE', 0)))
+
+        assert backbone_network is not None
         self.backbone_network = backbone_network
         try:
             self._docker_name = config.OTBR_DOCKER_NAME_PREFIX + str(nodeid)