JEST tests fail on React components as pure functions

After updating to a newer lint rule set, we converted some components into pure functions. This made the linting happy, but broke the JEST unit tests.

I did some debugging and found out that the pure functions were now returning NULL from the React TestUtils.renderIntoDocument() call. Changing that function back to a component made the JEST test pass, but made ESLINT unhappy.

I ended up removing the React renderIntoDocument call and using Enzyme‘s mount function instead. Voila! Even the pure functions now properly return an HTML structure, and the JEST tests pass.