Use the packagestest framework to test completion. Add support for a
slice of token.Position to packagestest to support this.
Change-Id: Ie5ddece4446a3c74419727461a77faa3788cb040
Reviewed-on: https://go-review.googlesource.com/c/148197
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
