Add utilities for shallow rendered components
Add utilities for shallow rendered components

We don't currently have a DOM available in our tests. Consequently we can't
query an element's structure or get its internal text using standard DOM API
calls. We can walk the structure using props.children accessors but this is
brittle and rather inelegant.

These utilities are intended to provide abstractions and keep the tests
concise and flexible.

Raffo/marathon-ui,quamilek/marathon-ui,cribalik/marathon-ui,mesosphere/marathon-ui,yp-engineering/marathon-ui,watonyweng/marathon-ui,janisz/marathon-ui,mesosphere/marathon-ui,janisz/marathon-ui,quamilek/marathon-ui,yp-engineering/marathon-ui,Raffo/marathon-ui,Kosta-Github/marathon-ui,Kosta-Github/marathon-ui,cribalik/marathon-ui,watonyweng/marathon-ui