Also add cmd/internal/test2json, the actual implementation,
which will be called directly from cmd/go in addition to being
a standalone command (like cmd/buildid and cmd/internal/buildid).

For #2981.

Change-Id: I244ce36d665f424bbf13f5ae00ece10b705d367d
Reviewed-on: https://go-review.googlesource.com/76872
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>