test: additional generic type switch test coverage

test: additional generic type switch test coverage

None of the current generic type switch test cases exercise type
switches where the instantiated case is an interface type.

Change-Id: I9272fa61b8dde1fe1a3702d524d4f40253ef19b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/390354
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>