Add cross-platform netstandard2.0 test coverage #1865

Job	Run time
build (macos-latest, Debug)	6h 0m 20s
build (ubuntu-latest, Debug)	5m 52s
build (windows-latest, Debug)	10m 36s
build (windows-latest, Release)	9m 55s
build (macos-latest, Release)	19m 0s
build (ubuntu-latest, Release)	5m 25s
publish-coverage	0s
	6h 51m 8s

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