go/internal/gcimporter: add unit tests for blank parameter names

go/internal/gcimporter: add unit tests for blank parameter names

cmd/compile internally rewrites blank return parameters to "~bN". Add
a test to make sure this isn't exposed via the go/types API.

Change-Id: I319644dc5adf483ed30520fd8e9d88cf5cea9751
Reviewed-on: https://go-review.googlesource.com/107616
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>