[dev.typeparams] test: update regress tests for types2

Followup to previous commit that extended test/run.go to run more
tests with -G=3. This CL updates a handful of easy test cases for
types2 compatibility.

Change-Id: I58a6f9ce6f9172d61dc25411536ee489ccb03ae0
Reviewed-on: https://go-review.googlesource.com/c/go/+/320610


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