cmd/compile/internal/types2: add missing tests from go/types

cmd/compile/internal/types2: add missing tests from go/types

Add 3 tests that exist in go/types but that were not ported to types2.

Change-Id: I34d219f605f9ae66e8b4439c3dfe93cfa0ec9524
Reviewed-on: https://go-review.googlesource.com/c/go/+/501304
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>