ci: run integration tests on all three OSes #239

Job	Run time
unit-test (ubuntu-latest)	26s
unit-test (macos-latest)	50s
unit-test (windows-latest)	1m 9s
build	24s
integration-test (ubuntu-latest)	1m 2s
integration-test (windows-latest)	1m 57s
integration-test (macos-latest)	1m 41s
	7m 29s

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