reflect: add test for interface type NumMethod

reflect: add test for interface type NumMethod

While at it, also use concrete type for non-interface type test.

Change-Id: Ie468c30ee31ba99ef8f9a810d3be851fd37b9b43
Reviewed-on: https://go-review.googlesource.com/c/go/+/478356
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>