isolate tests in child processes #159

Job	Run time
build (ubuntu-latest, Debug, address)	26s
build (ubuntu-latest, Debug, memory)	12s
build (ubuntu-latest, Debug, undefined)	12s
build (ubuntu-latest, RelWithDebInfo, address)	7s
build (ubuntu-latest, RelWithDebInfo, memory)	17s
build (ubuntu-latest, RelWithDebInfo, undefined)	2s
build (ubuntu-latest, MinSizeRel, address)	12s
build (ubuntu-latest, MinSizeRel, memory)	8s
build (ubuntu-latest, MinSizeRel, undefined)	13s
build (ubuntu-latest, Release, address)	9s
build (ubuntu-latest, Release, memory)	13s
build (ubuntu-latest, Release, undefined)	6s
build (macOS-latest, Debug, address)	9s
build (macOS-latest, Debug, undefined)	9s
build (macOS-latest, RelWithDebInfo, address)	1s
build (macOS-latest, RelWithDebInfo, undefined)	1s
build (macOS-latest, MinSizeRel, address)	9s
build (macOS-latest, MinSizeRel, undefined)	9s
build (macOS-latest, Release, address)	8s
build (macOS-latest, Release, undefined)	1s
	3m 4s

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