test: add bug494: test case miscompiled by gccgo

Gccgo incorrectly executed functions multiple times when they appeared
in a composite literal that required a conversion between different
interface types.

Change-Id: I7b40e76ed23fa8440ffa03b262041265c109adf7
Reviewed-on: https://go-review.googlesource.com/1710
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>