plugin text syntaxes: remove side effect on init: When text syntaxes was initialised, `syntaxes` dict was filled, but it was a class attribute. As a result, re-initialising it (which may happen for tests, and maybe in the future dynamically), was not starting with an empty dict, but with an already-filled one, resulting in conflicts. This patch fixes it by moving `syntaxes` to an instance attribute.

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)