[dev.typeparams] test: skip -G=3 testing under GOEXPERIMENT=unified
[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 <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Trust: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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