go/printer: idempotent comment formatting

go/printer: idempotent comment formatting

Also:

- Refactored testing framework to permit easier
idempotency testing.

- Applied gofmt -w src misc

This CL depends on CL 6639044 being applied first.

Formatting is not idempotent for all files: In those
files the comment position has changed (due to missing
precise location information) and/or the comment formatting
cannot/is not aware of independent code re-formatting.
In general it is very hard to make format idempotent when
running it in one pass only. Leaving that aside for now.

Fixes #1835.

R=r, rsc
CC=golang-dev
https://golang.org/cl/6624051