reflect: Add tests for Call with functions taking and returning structs.

reflect: Add tests for Call with functions taking and returning structs.

gccgo has problems using reflect.Call with functions that take and
return structs with no members.  Prior to fixing that problem there, I
thought it sensible to add some tests of this situation.

Update #6761

First contribution to Go, apologies in advance if I'm doing it wrong.

R=golang-dev, dave, minux.ma, iant, khr, bradfitz
CC=golang-dev
https://golang.org/cl/26570046