Add some basic tests for diagnostics using the new
go/packages/packagestest framework.
Change-Id: I6a7bfba6c392928a9eb123ab71ceb73785c12600
Reviewed-on: https://go-review.googlesource.com/c/145697
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>