[test-proxy] Test proxy adds `traceparent` header

It appears that `test-proxy` is adding a `traceparent` header that was not present in the incoming request (the `Content-Length` header is covered by #4239):

```
GET http://localhost:6000/ HTTP/1.1
Host: localhost:6000
traceparent: 00-592e265ff633f18374c8e4705fadcfc6-cbabc87d52c25215-00
Content-Length: 0
```

Servers will likely ignore this header, but technically `test-proxy` should not be adding it.