provide React 18 test infrastructure for BTOG

Summary: Copy and export the react-agnostic functions provided by @bigtest/react.

Details: @bigtest/react has four sets of interrelated functions used in test-setup. Some are react-agnostic, and some rely on react v17, specifically the deprecated ReactDOM.render API. In order to continue using BTOG with react v18, we want to make the react-agnostic functions available for import without being harassed by compatibility warnings since @bigtest/react technically depends on v16.