go.tools/go/types: add test case for resolution of blank methods

(was bug on go/types wishlist; probably fixed a while ago)

R=adonovan
CC=golang-dev
https://golang.org/cl/13633047

go/types/resolver_test.go

@@ -53,6 +53,12 @@ var sources = []string{
 		}
 	}
 	`,
+	`
+	package p
+	type S struct{}
+	func (T) _() {}
+	func (T) _() {}
+	`,
 }
 
 var pkgnames = []string{