design/2981: coverage
design/2981: coverage

Added coverage result to JSON output

Renamed {Benchmark,Test}Result.Output to Log, to make it even more
obvious.

Specified that examples are treated as tests.

Change-Id: I80ae1c813ea21508a9f2093fb5e1ea586fc2fa8a
Reviewed-on: https://go-review.googlesource.com/15704
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

golang/proposal,golang/proposal