Custom EUI jest snapshot serializer

[JasonStoltz]

Upgrading the EUI library in Kibana can be cumbersome, as many of our components are included in snapshot tests.

We proposing that we build a custom snapshot serializer for EUI components. The goal is to avoid heavy churn when making updates in Kibana.

[cee-chen]

Closed by #6769. At this time, this isn't an approach we can feasibly take due to LOE (although that may change in the future if Enzyme usage drops). We could investigate instead encouraging or evangelizing Kibana teams to avoid snapshot usage.