Use @testing-library/react in useMutation-test.js

Summary: Refactors useMutation-test from ReactTestRenderer to ReactTestingLibrary. This is mostly a proof of concept / test plan for the stack. But we can also land it to check off one module on the migration away from RTR.

Reviewed By: yungsters

Differential Revision: D63323949

fbshipit-source-id: 46801f0aecd6b5155515e5085194247d6aef16b3