[choice-group] check multipleChoice change detection

[tlouisse]

Expected behavior

changedProperties in updated should contain modelValue only when array contents differ

Actual Behavior

It will probably fail on case 1 below.

Make a test that reflects #1026 (comment)

Assume we have a group with checkedIndex [0,1,4]
So spy on changedProperties and see if modelValue detects a change when:

1. we do this.checkedIndex = [0,1,4] -> array ref changed, although we DO NOT expect it to be picked up in updated
2. we do this.checkedIndex.push(2) -> array ref didn't change, but we DO expect it to be picked up in updated

[tlouisse]

Problem: checking a single-choice child always triggers 2 events:

• the newly checked option event
• the unchecked event of the previously checked option
  Fixing this requires to evaluate sending the model-value-changed event asynchronously, and then we can compute our values based on all children events (the test above would run).
  But... by doing this (evaluating async) related select-rich / listbox tests that depend on synchronicity fail. It might be breaking in other contexts as well.

So it’s better to schedule till a refactor for v1: make parent group responsible for all bookkeeping, without waiting for children
events. This will be better for readibility, maintainability and performance.

See this example for how this could work: https://github.com/ing-bank/lion/blob/feat/manyMenus/packages/menu/src/InteractiveListMixin.js#L574

Test we could use in the future:

 it('only fires model-value-changed when selected indexes differ', async () => {
  const el = await fixture(html`
  <${tag} multiple-choice>
    <${optionTag} .choiceValue=${'10'} checked>Item 1</${optionTag}>
    <${optionTag} .choiceValue=${'20'}>Item 2</${optionTag}>
    <${optionTag} .choiceValue=${'30'}>Item 3</${optionTag}>
  </${tag}>
`);
  const spy = sinon.spy(() => {});
  el.addEventListener('model-value-changed', spy);
  expect(el.checkedIndex).to.eql([0]);
  el.checkedIndex = [0];
  await el.updateComplete;
  expect(spy).to.not.have.been.called;
  el.checkedIndex = [1];
  await el.updateComplete;
  expect(spy).to.have.been.calledOnce;
});