test: allow to run tests in parallel #210

Job	Run time
build (18.x, ubuntu-latest)	34s
build (18.x, windows-latest)	2m 26s
build (18.x, macos-latest)	1m 1s
build (20.x, ubuntu-latest)	20s
build (20.x, windows-latest)	1m 22s
build (20.x, macos-latest)	28s
test_20 / test (18.x, ubuntu-latest)	3m 21s
test_20 / test (18.x, windows-latest)	4m 51s
test_18 / test (18.x, ubuntu-latest)	3m 6s
test_20 / test (18.x, macos-latest)	3m 11s
test_18 / test (18.x, windows-latest)	5m 20s
test_host / test (18.x, ubuntu-latest)	5m 27s
test_20 / test (20.x, ubuntu-latest)	2m 57s
test_18 / test (18.x, macos-latest)	3m 8s
test_18 / test (20.x, ubuntu-latest)	3m 4s
test_host / test (18.x, windows-latest)	3m 33s
test_20 / test (20.x, windows-latest)	4m 45s
test_18 / test (20.x, windows-latest)	5m 23s
test_host / test (18.x, macos-latest)	1m 4s
test_20 / test (20.x, macos-latest)	3m 21s
test_host / test (20.x, ubuntu-latest)	5m 53s
test_18 / test (20.x, macos-latest)	3m 1s
test_host / test (20.x, windows-latest)	5m 38s
test_host / test (20.x, macos-latest)	1m 16s
	1h 14m 30s

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