pkg/manager/diff: add tests #19071

Job	Run time
race_dashboard	4m 18s
build	3m 10s
dashboard	2m 14s
old	7m 8s
aux	4m 37s
race	9m 28s
arch (presubmit_arch_windows)	3m 35s
gvisor	4m 8s
arch (presubmit_arch_netbsd)	4m 8s
arch (presubmit_arch_openbsd)	3m 49s
arch (presubmit_arch_freebsd)	6m 31s
arch (presubmit_arch_linux)	11m 2s
arch (presubmit_arch_executor)	3m 23s
arch (presubmit_arch_darwin)	3m 51s
	1h 11m 22s

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