Wait for in-flight message handlers before ProcessMessagesCoreAsync r… #2393

Job	Run time
build (ubuntu-latest, Debug)	13m 20s
build (windows-latest, Release)	12m 25s
build (macos-latest, Release)	7m 24s
build (macos-latest, Debug)	8m 22s
build (ubuntu-latest, Release)	7m 10s
build (windows-latest, Debug)	10m 49s
publish-coverage	0s
	59m 30s

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