Add new event test for frameworks breaking event bubbling.

[ChrisShank]

Some component frameworks (e.g. react, svelte 5, solid) use "synthetic event delegation" to invoke event handlers. When used in conjunction with custom elements that attach event handlers outside of these synthetic event systems this will subtly break usage ofevent.stopPropagation(). The proposal here is to add a new test to detect when this happens and ding component frameworks that don't comply.

This issue is spurred by a conversation with @KonnorRogers and @lukewarlow on twitter.

Examples of this in action(not to pick on anyone in particular):

• Svelte breaks

• Vue works fine