[spector] add tests for `--namespace` flags

[iscai-msft]

1. --namespace is the not set and no @clientNamespace used
2. @clientNamespace is used for the root namespace in the spec, but no --namespace in tspconfig
3. Only --namespace is used and no @clientNamespace used
4. --namespace is used but the spec has one root namespace that is renamed with @clientNamespace
5. --namespace is used and @clientNamespace is specified for a single model
6. --namespace is used and @clientNamespace is specified for a sub-namespace containing a client and some models. Outside this sub-namespaces, there can be other clients and models.