Properly terminate auxiliary processes when test exits #121

Job	Run time
test / matrix-prep-bazelversion	7s
test / matrix-prep-os	4s
test / test (ubuntu-latest, 7.3.1, ., true)	12m 34s
test / test (ubuntu-latest, 8.0.0, ., true)	13m 1s
	25m 46s

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