Make extra_artifacts check optional

Make extra_artifacts check optional

There is an ongoing debate about the usefulness of the extra artifacts
check. While it might be useful to detect unexpected behaviour in some
tests, it feels extraneous in many cases. This change provides a middle
ground by making the artifact checking optional. This might be
especially useful for writing new tests, since the author gets to decide
whether the check is useful -- and can utilize it, or can skip it for
sake of brevity.