cmd/go: add -testsum flag to update go.sum in script tests

-testsum may be set to "tidy", "listm", or "listall". When set,
TestScript runs 'go mod tidy', 'go list -m -mod=mod all', or
'go list -mod=mod all' at the beginning of each test that has a go.mod
file in its root directory. If the test passes and go.mod or go.sum
was updated, TestScript will rewrite the test file with the initial
content of go.mod and go.sum (after the above command).

This is useful for writing tests that need a working go.sum and for
fixing tests that rely on -mod=mod.

For golang/go#41302

Change-Id: I63a5667621a5082ccedfc1bff33c3969c29e8b3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/336150


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/341932


Reviewed-by: Bryan C. Mills <bcmills@google.com>