Try converting integration tests to Jest

[gaearon]

We are currently using Mocha because there was some jsdom issue when we tried to run them with Jest. It's worth trying to convert them to Jest again and seeing if it works.

The tests are called here:

https://github.com/facebookincubator/create-react-app/blob/ae0cb23dc927e1d47c026b8d801cb2a2328822e9/tasks/e2e-kitchensink.sh#L175-L187

and are declared here:

https://github.com/facebookincubator/create-react-app/tree/master/packages/react-scripts/fixtures/kitchensink/integration