We now have pretty good support for users of cgo packages. Add tests.
Closesgolang/go#35720.
Change-Id: Icdc596038bc6fca1c08eacd199def12264cf512d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/208503
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>