pkg/manager/diff: add tests #19070

Job	Run time
race	7m 14s
gvisor	4m 13s
dashboard	2m 7s
race_dashboard	4m 5s
build	3m 6s
old	6m 57s
aux	3m 56s
arch (presubmit_arch_freebsd)	6m 32s
arch (presubmit_arch_windows)	3m 38s
arch (presubmit_arch_linux)	10m 35s
arch (presubmit_arch_netbsd)	4m 3s
arch (presubmit_arch_openbsd)	4m 1s
arch (presubmit_arch_executor)	3m 24s
arch (presubmit_arch_darwin)	3m 53s
	1h 7m 44s

Provide feedback