ci enhancement: scan hub logs for warnings/error that shouldn't happen

[consideRatio]

We would benefit from some test that runs in the end of all tests and observes the hub pod's logs for various patterns.

For example this entry should cause a failure I'd say.

[W 2021-09-26 00:36:57.830 JupyterHub configurable:193] Config option `load_role` not recognized by `JupyterHub`.  Did you mean one of: `load_groups, load_roles`?

Related

I opened ipython/traitlets#677 about having traitlets itself raise an error on such warning.