Also: Fix (testdata/gen/) copyGen.go, zeroGen.go, and arithConstGen.go
to actually match (testdata/) copy.go, zero.go, and arithConst.go, all
of which were manually edited in https://go-review.googlesource.com/20823
and https://go-review.googlesource.com/22748 despite the 'do not edit'
(or perhaps because it was missing in the case of arithConst.go).

For #13560.

Change-Id: I366e1b521e51885e0d318ae848760e5e14ccd488
Reviewed-on: https://go-review.googlesource.com/39172Reviewed-by: Rob Pike <r@golang.org>