I don't think this comment really adds much more information than the code already provides, because it's describing what the code does instead of why it does it. I think we can make this comment more helpful by explaining the intention here, like you did in the PR description, e.g.

// Allow children to re-render.

Do we need this Promise? I tested it without it and the test seemed to work normally. Maybe I'm missing something?

Also, I found this test a little difficult to follow because of the indirection added by getItem and makeItems. I think if we hard-coded the items we're passing in then this code will be become more direct (and thereby more readable, at least to me 😅). It becomes a little more difficult to change, but I think this code will be read much much more often than its read, mostly because these parts of the tests probably won't change for a long time. WDYT?

      it(`should not re-render if any items's watchedItemProps did not change`, () => {
        const items = [(
          <EuiContextMenuItem key="A" data-counter={0}>
            Option A
          </EuiContextMenuItem>
        ), (
          <EuiContextMenuItem key="B" data-counter={1}>
            Option B
          </EuiContextMenuItem>
        )]

        // by not passing `watchedItemProps` no changes to items should cause a re-render
        const component = mount(
          <EuiContextMenuPanel
            items={items}
          />
        );

        expect(component.debug()).toMatchSnapshot();

        component.setProps(
          { items: makeItems() },
          () => {
            expect(component.debug()).toMatchSnapshot();
          }
        );
      });

Same comment applies here and to the other test.

I forgot to mention this before, you can import takeMountedSnapshot from the test module on line 4, and use it here like this: expect(takeMountedSnapshot(component)).toMatchSnapshot();, which will yield a snapshot of regular HTML:

<div
  class="euiContextMenuPanel"
  tabindex="0"
>
  <div>
    <button
      class="euiContextMenuItem"
      data-counter="0"
      type="button"
    >
      <span
        class="euiContextMenu__itemLayout"
      >
        <span
          class="euiContextMenuItem__text"
        >
          Option A
        </span>
      </span>
    </button>
    <button
      class="euiContextMenuItem"
      data-counter="1"
      type="button"
    >
      <span
        class="euiContextMenu__itemLayout"
      >
        <span
          class="euiContextMenuItem__text"
        >
          Option B
        </span>
      </span>
    </button>
  </div>
</div>

It's definitely not necessary to use this in these tests, but I thought I should mention it!

The tests still execute both assertions if this line is removed. Is this a defensive measure in situations where setProps is asynchronous? I did some digging to see if this was the case but Enzyme's docs don't mention anything about sync vs async, so this is new ground for me.