pkg/manager/diff: add tests #19108

Job	Run time
gvisor	3m 57s
aux	4m 8s
arch (presubmit_arch_netbsd)	4m 21s
arch (presubmit_arch_openbsd)	3m 50s
race_dashboard	3m 32s
dashboard	2m 27s
arch (presubmit_arch_linux)	10m 57s
race	9m 17s
build	3m 51s
old	7m 15s
arch (presubmit_arch_freebsd)	6m 56s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_darwin)	3m 50s
arch (presubmit_arch_executor)	3m 23s
	1h 11m 20s

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