cmd/compile/internal/types2: use self_test.go from go/types

This CL replaces self_test.go with the (improved) version
from go/types, modified for types2.

To see the differences between go/types/self_test.go and
this version, compare against patch set 1.

Change-Id: I7ae830a17f7a0de40cc1f5063166a7247f78ec27
Reviewed-on: https://go-review.googlesource.com/c/go/+/300997


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>