[dev.typeparams] cmd/compile/internal/types2: disable external test temporarily

This is making rf unhappy for extremely subtle reasons,
so for now just disable the external test here. Will reenable
once the big rewrite is done.

Change-Id: Ifd1ba95e2843792427629d1660850fe531cdd0b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/279531


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>