design/2981: -json flag in go test
design/2981: -json flag in go test

Add `-json` flag to `go test`.
When it is passed, `go test` stdout is valid indented JSON format.

Add `-stream` flag to `go test` to explicitly specify that multiple JSON
objects in stdout is OK.

Change-Id: Ib526e8fe3eeef641c9015af93848755669b58830
Reviewed-on: https://go-review.googlesource.com/15554
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/proposal,golang/proposal