cmd/go: go test -json should only print JSON to stdout

[rsc]

Per discussion on #53893, go test -json should only print JSON to stdout.
We should ensure that's true.

[seankhliao]

see also #35169

[gopherbot]

Change https://go.dev/cl/529120 mentions this issue: cmd/go: avoid writing non-JSON "build failed" errors from 'go test -json'

[matloob]

I think this is now fixed. It looks like we write everything using the test2json converter for stdout or using the build json printer if gotestjsonbuildtext is not set.