GitLab

TestMakeFuncVariadic only called the variadic function via Call and
CallSlice, not via a direct function call.

I thought these tests would fail under gccgo tip, but they don't.  Still seems worth having though.

LGTM=iant
R=golang-codereviews, gobot, iant
CC=golang-codereviews
https://golang.org/cl/152060043