fix flaky participation_test.exs

It was tested if the updated_at after marking as "read" was equal as the updated_at at insertion, but that seems wrong. Firstly, if a record is updated, you expect the updated_at to also update. Secondly, the insert and update happen almost at the same time, so it's flaky regardless.

Here I make sure it has a much older updated_at during insert so we can clealy see the effect after update. I also check that the updated_at is actually updated because I expect that this is the expected behaviour and it's also the current behaviour.