testenv.StandIns (06420a71) starts each stand-in once in the background on
macOS and records it with os.Stat, which reads time.Local. internal/
sessions' inZone, internal/usage's TestDayFilterDST and the root package's
sessions CLI setup assigned time.Local while those runs could still be
under way. `go test -race -run '^TestDsh$' ./internal/sessions` reported
the race 6 of 6 runs on main (stats_test.go:14 against program.go:140).
testenv.Zone(t, loc) waits on the same WaitGroup RemovePrograms waits on,
sets time.Local and restores it in a Cleanup; the three helpers use it.
The root setup registers Zone before its own Cleanup, so the zone still
goes back after sessions.Reset's index write, as before.
After: the same command 0 races in 6 runs; -race -count=20 of
Dsh|DST over sessions and usage, -race -count=3 of the root Sessions
tests, -race of sessions/testenv/usage/root, go vet, gofmt pass. Tests only.