[dev.typeparams] test: skip -G=3 testing under GOEXPERIMENT=unified

In normal build configurations, we test both -G=0 and -G=3 so that we
can test both typecheck and types2. However, GOEXPERIMENT=unified
always uses types2, so testing both is redundant.

Change-Id: I697d2ad916d8b17cfaf4f0b6b32eec380d4e7906
Reviewed-on: https://go-review.googlesource.com/c/go/+/330755


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>