Fix XunitLoggerProvider crash when background work logs after test disposal #1844

Job	Run time
build (ubuntu-latest, Debug)	6m 45s
build (windows-latest, Release)	11m 0s
build (macos-latest, Debug)	5m 25s
build (windows-latest, Debug)	13m 17s
build (macos-latest, Release)	7m 8s
build (ubuntu-latest, Release)	6m 7s
publish-coverage / publish-coverage	34s
	50m 16s

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