reflect: add tests for variadic method calls
reflect: add tests for variadic method calls

These tests fail when using gccgo.  In gccgo using Interface
on the value of a method function is implemented using a
variant of MakeFunc.  That approach did not correctly handle
variadic functions.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/151280043

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old