Users of packagestest will create modules that don't exist on the
Internet and can change. There's no point in checking sum.golang.org for
them under any circumstances.
Similarly for the various goimports tests.
Fixesgolang/go#32216.
Change-Id: Id9a6b660564cb744530bf9d209fca19008fb9c4f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/178722
Reviewed-by: Ian Cottrell <iancottrell@google.com>