Improve async-related React tests

[chrisronline]

We had this weird code supporting our React tests that contained async mounting logic (see here).

However, after digging into #16895 with @cjcenizal, the comments or issue didn't help me understand what was happening so I dug in more and found a simpler solution that I think will make this easier to maintain and understand moving forward.

We can replace the three await component.update() calls with:

// Ensure all promises resolve
await new Promise(resolve => process.nextTick(resolve));
// Ensure the state changes are reflected
component.update();

I think this makes more sense and should be more helpful for people looking at this in the future.

cc @jen-huang @bmcconaghy

[cjcenizal]

Nice! It might be worth extracting part or all of this into a helper function in the test module.

[timroes]

I will close this as too specific right now, and there are no short-term plans to implement this.