test: migrate editors-non-ideal-state test to RTL

[erikian]

Ref: #1408.

This is easier to review one commit at a time since Github is not picking up the move across folders.

I've refactored renderNonIdealState so that we render it as JSX (<RenderNonIdealState editorMosaic={editorMosaic} />) instead of calling it as a regular function (renderNonIdealState(editorMosaic)) like we were doing.

[coveralls]

coverage: 87.602%. remained the same
when pulling 5c8cb85 on rtl/editors-non-ideal-state
into 3228c9c on main.