+ also fix selectedValue not using defaultValue, even though the input does

e.g. booleans, numbers, etc.

…own menu

- to reduce UX confusion when options are shown in the dropdown but not the input

nb: propTypes throws a dev error on this, but IMO it's better for the end-user to just not render the option at all

- convert from Enzyme to RTL
- DRY out open dropdown logic
- make the initial it renders test capture the portalled dropdown - no need to make a separate test for that
- remove unnecessary portal mock
- replace snapshots with specific assertions where it makes sense
- remove unnecessary `onChange={() => {}}` and write an actual onChange test
- remove other props if they're not actually being used by the test

`= []` will cause a new instantiation every rerender, which makes the useMemo useless - just optional chaining instead

This reverts commit 344e430.