go/types,cmd/compile/internal/types2: fix TestStdlib

go/types,cmd/compile/internal/types2: fix TestStdlib

CL 402556 moved a special-cased package and broke the longtests.

Change-Id: I10b48e0912c1cee0eabc342ed1c1704ed79946bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/404474
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Auto-Submit: Filippo Valsorda <valsorda@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>