[dev.typeparams] go/types: convert testdata/check tests to type set syntax

This is a port of CL 324569 to go/types, with some error positions
adjusted: go/types puts errors related to terms on the '~', not type
name.

Change-Id: I92e8443ce27a5ecae0e3e0dac4811eaf3eee07ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/326682


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>