Add tests for <select multiple> => <select> transition

[gaearon]

I deleted it in #10281 because it wasn’t testing the public API, and it was hard to say if it tested the path the code actually took.

We should either add a similar test against public API or an equivalent test to manual fixtures.
See #10281 (comment) for details.

[raunofreiberg]

This should be tested now.

react/packages/react-dom/src/__tests__/ReactDOMSelect-test.js

Line 337 in de5102c

it('should allow switching from multiple', () => {

[nhunzaker]

So it is, 👍 thanks!