cmd/go/internal/modfetch: add zip sums and hashes to TestCodeRepo

Updates #35290

Change-Id: I09cad17f09e78c2bf6a9de98b01f13ed383ca006
Reviewed-on: https://go-review.googlesource.com/c/go/+/204643
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod