[dev.typeparams] cmd/compile/internal/syntax: convert (most) parser tests to new type set syntax

Left a couple of tests with old notation so that we keep testing it
while the notation is still supported.

Change-Id: Ia6a3e7911af87eaccc7b06189c10f79789575a98
Reviewed-on: https://go-review.googlesource.com/c/go/+/328256


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