Add focus middleware mock and tests

[nicknisi]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

This adds a focus middleware mock that is capable of mocking the return value of the middleware's isFocused method.

const focusMock = createFocusMock();
const factory = create({ focus });
const App = factory(({ middleware: { focus } }) => {
	return <div key="root">{focus.isFocused('root') ? 'focus' : 'no focus'}</div>;
});
const h = harness(() => <App />, { middleware: [[focus, focusMock]] });

focusMock('root', false);
h.expect(() => <div key="root">no focus</div>);

focusMock('root', true);
h.expect(() => <div key="root">focus</div>);

Resolves #627

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