cmd/go: avoid writing non-JSON "build failed" errors from 'go test -json'

cmd/go: avoid writing non-JSON "build failed" errors from 'go test -json'

In 'go test -json' we expect stdout to contain only JSON events,
not unstructured text. Unstructured text should either go to stderr
or be wrapped in a JSON event.

(If we add structured build output in #62067, we can emit this output
as a build event instead of a test event.)

Fixes #35169.
For #54378.

Change-Id: Ibedd28e79b5adf8d6ae56165b9f0393b14ece9aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/529120
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>