Route table check tool coverage calculation incorrect when using cluster_header

*Title*: *Route table check tool coverage calculation incorrect when using cluster_header in a route config*

*Description*:
When a configuration contains multiple routes which utilise cluster_header for selecting a cluster the route tester does not calculate coverage properly. In the example below you see 50% coverage reported when both routes exist yet isolating each route and test reports 100%. This is particularly an issue when using `--fail-under` as part of a pipeline. If you switch to using `cluster` rather than `cluster_header` the problem goes away.

In case anyone experiences this a workaround is to replace the `cluster_header` node with `cluster` and a fixed value when running the tests. A tool such as yq can do this.

*Repro steps*:
Take the following Envoy config and tests as an example.

Envoy route table check tool version 1.21.4

```
virtual_hosts:
- name: api_domain
domains: ["test.com"]
routes:
- match:
prefix: /test1
route:
cluster_header: "x-cluster"

- match:
prefix: /test2
route:
cluster_header: "x-cluster"
```

```
tests:
- test_name: test1
input:
authority: test.com
path: /test1
method: GET
additional_request_headers:
- key: x-cluster
value: "cluster_a"
validate:
cluster_name: cluster_a

- test_name: test1
input:
authority: test.com
path: /test2
method: GET
additional_request_headers:
- key: x-cluster
value: "cluster_a"
validate:
cluster_name: cluster_a
```

Running the route test will return 50% coverage when the expected is 100%. Now, remove the first or second route and matching test and see 100% coverage is returned. Doing the same with the other route/test will also return100%. Replacing `cluster_header ...` with `cluster: cluster_a` in either route will also return 100%.

Luckily this is deterministic so repro should not be an issue.