go.tools/go/types: add test cases for parenthesized receivers
go.tools/go/types: add test cases for parenthesized receivers

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/101510043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools