Fix flaky test: add sync point in ReadEventsAsync_InStreamingMode_YieldsNewlyWrittenEvents #2609

Job	Run time
build (ubuntu-latest, Release)	13m 11s
build (macos-latest, Release)	7m 43s
build (windows-latest, Debug)	3m 44s
build (windows-latest, Release)	3m 27s
build (macos-latest, Debug)	4m 41s
build (ubuntu-latest, Debug)	12m 51s
publish-coverage	0s
	45m 37s

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