It also improves the error in case it's the first time we are running
the tests and we have no coreNLP service running.

Added more tests for more subtle cases.

(no default getCurrentTime anymore)

This is a first stepping stone in avoiding bugs by having `UserId` and
`ContextId` being proper types, and not synonyms.

We also refactor things to have `AuthenticatedUser` carry the `UserId`.

Tests were written with a very flat module hierarchy which made them
a bit confusing to navigate. This commit moves everything under a common
`Test.*` hierarchy.