[bug]: `TestPropForEachConcOutperformsMapWhenExpensive` failed

[yyforyongyu]

From this build,

--- FAIL: TestPropForEachConcOutperformsMapWhenExpensive (0.49s)
    slice_test.go:341: #65: failed on input 3468287706918430388, []int{5147038937881347842, 6791312966902822189}
FAIL
FAIL	github.com/lightningnetwork/lnd/fn/v2	1.767s
FAIL