Also be clear that go test output is not suitable for piping into test2json.

Fixes #22710.
Fixes #22789.

Change-Id: I3d850c8a2288be7f9a27d638bbf847cb8707dcce
Reviewed-on: https://go-review.googlesource.com/81555
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>