Improve forgot-await detection by tracking async calls

[calebeby]

When @spaceninja and I were working on some tests, we found a bug where the forgot-await detection didn't work:

const buttonEl = await screen.getByRole("button", { name: /count/i });
await expect(buttonEl).toHaveTextContent(/count is: 0/i);
user.click(buttonEl); // Should trigger forgot-await error
await expect(buttonEl).toHaveTextContent(/count is: 3/i);

This is because the current implementation usually only works if the call with await missing is the last call inside withBrowser, because it depends on Jest's feature of listening for unhandled promise rejections.

This PR reimplements the forgot-await detection. Now it is centralized, and it works differently. It works by keeping track of all async calls that are started inside withBrowser. When withBrowser finishes, it checks to see if any of the calls did not finish yet. If the developer remembered to use await on all the calls, then all the calls should have already finished, so nothing happens. If the developer forgot await on one or more of the calls, withBrowser can see that, and throw the error.

This does not handle the case where the call that is missing await executes fast enough that it finishes before the rest of the things in withBrowser.

Review with whitespace changes turned off, otherwise the diff is a lot bigger than it needs to be

[gerardo-rodriguez]

Reviewing this now. Gotta love the tests, they are always the most helpful to understand the intent of the changes. 🙂

[gerardo-rodriguez]

LGTM! I didn't see anything that obviously jumped out at me. Great work, @calebeby! 🎉