Add regression tests where sync render causes later concurrent render to expire soon

[jddxf]

Currently we only reset currentEventTime when we start concurrent render. So currentEventTime is retained after sync render. That may make later concurrent render expire soon if enough time has elapsed since the last assignment to currentEventTime.

I've thought about resetting currentEventTime in ensureRootIsScheduled when there's nothing to work on the root. But that breaks the test below

react/packages/react-reconciler/src/__tests__/ReactExpiration-test.js

Lines 111 to 113 in 9620324

	it(
	'two updates of like priority in the same event always flush within the ' +
	"same batch, even if there's a sync update in between",

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[gaearon]

This seems to pass now. I'll merge.