Verify that versions used in hub and user env corresponds to our expectations

[consideRatio]

In dedbugging https://github.com/jupyterhub/the-littlest-jupyterhub/issues/885 we became uncertain about what version of jupyterhub was actually used.

We believed that jupyterhub 4 was used before we had upgraded to it. I think we concluded that we used jupyterhub 3 in the hub env, but maybe not in the user env. Currently we have no tests about ensuring we have the expected jupyterhub in the user env for example.

Action point

• Clarify expectations on versions in various envs on first install of tljh and after upgrades
• Test expectations

Drafted expectations

If #890 is merged in its current form, my expectations are that:

• after install or upgrade and hub env and user env, jupyterhub always the latest available minor/patch of the pinned major version
• after install, misc extra things are installed in the user env