test: README sections referenced by HelpError exist
Description
We have a HelpError (more info). This MR adds a test that makes sure that the HelpError always references existing README sections. Before this test the slugs in our code could get out of sync with the README.md
We already had the test in constants.test.ts
I moved it to the HelpError
so it's clearer that we are already testing the slug presence.
Edited by Tomas Vik