go/types, types2: establish type-checker specific local tests

go/types, types2: establish type-checker specific local tests

Establish testdata/local directory for tests that are not (yet)
shared between the go/types and types2. Move issue47996.go into
that directory.

For #54511.

Change-Id: I274b119990a93cc3657bdddc5246948699226c21
Reviewed-on: https://go-review.googlesource.com/c/go/+/426660
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>