cmd/compile/internal/types2: use self_test.go from go/types
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 <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go