Rewrite brittle copy-pinned tests as behavior contracts #2328

Job	Run time
build	13s
test (3/4)	55s
static-analysis	34s
test (1/4)	1m 27s
test (2/4)	44s
test (4/4)	40s
test (one-process pollution detector)	-1s
typecheck	3s
eslint	3s
prettier	4s
test (src)	4s
build-and-test	2s
	4m 48s

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