reflect: add a test for Calling a Method of a direct interface type

reflect: add a test for Calling a Method of a direct interface type

Gccgo's implementation of direct interface types has bugs that
causes reflect Call of method from Type.Method fail. CL 175837
and CL 175798 fix the bug. This CL adds a test.

Change-Id: I4e5f2cb96304c1ac7be04ca6d2851bac52b8eb24
Reviewed-on: https://go-review.googlesource.com/c/go/+/175880
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>