Also convert the format tests to use it. This means that the build bots no
longer need to run gofmt.
Change-Id: I5cb9d239183b17d81fdb00b38e9099d224c07e6a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/172973
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
