cmd/go: in tests, populate go.sum files and reduce usage of -mod=mod

[jayconrod]

This issue acts as a TODO for a number of comments raised in review of CL 251881, which enables -mod=readonly by default for #40278. Two broad issues should be fixed in the cmd/go integration tests.

First, -mod=mod is used in a lot of places with go list or go build. We should make the tests follow more idiomatic usage. If something is missing from go.mod or go.sum, we should run go get -d, go mod tidy, or go mod download first. The following tests need changes. There are probably more.

• mod_doc
• mod_download.
• mod_invalid_version
• mod_list
• mod_load_badzip
• mod_replace
• mod_replace_gopkgin
• mod_require_exclude
• mod_sumdb_golang

Second, many tests fail without -mod=mod because the test archive doesn't include a go.sum file. I'd rather not include literal go.sum files in tests, since they'll make tests more difficult to write and act as change detectors. Instead, we should have a command or sequence of commands run at the beginning of a test that fills in missing sums without modifying go.mod. Something like:

cp go.mod go.mod.orig
go get -d ./...
cmp go.mod go.mod.orig

If this turns out to be too repetitive, we may want to add a script_test.go command to shorten it.

The following tests need changes. Again, there are probably more.

• mod_list_dir
• mod_list_replace_dir
• mod_list_upgrade
• mod_load_badmod
• mod_modinfo

[jayconrod]

Also, mod_missingpkg_prerelease. go get -d should report errors on transitively imported packages.

[gopherbot]

Change https://golang.org/cl/311189 mentions this issue: cmd/go: in tests, reduce usage of -mod=mod and populate go.sum files

[gopherbot]

Change https://golang.org/cl/336150 mentions this issue: [dev.cmdgo] cmd/go: add -testsum flag to update go.sum in script tests

[gopherbot]

Change https://golang.org/cl/341932 mentions this issue: cmd/go: add -testsum flag to update go.sum in script tests