Make go.sum a pre-requisite of tests/builds.

As it stands, trying to run tests/build with a `go.sum` that is older than `go.mod` produces the error below:

```console
go test ./...
go: updates to go.mod needed; to update it:
	go mod tidy
```

Possible recipe:

```Makefile
go.sum: go.mod
	go mod tidy
```