cmd/go: test that graph, verify, and why don't write go.mod or go.sum
They should also not report an error if these files need to be
updated. These commands are used for debugging, so it's important that
they still work when go.mod and go.sum are incomplete.
For #40775
Change-Id: I1b731599e5a4510f47827b9812525636a7402bf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/351468
Trust: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>