[dev.typeparams] cmd/compile/internal/syntax: add type parameter tests

The file endings are not .go so that gofmt leaves these files alone.
They are also not .src to distinguish them from regular go source tests.

Change-Id: I741f5c037bad1ea9d6f7fda3673487d0be631350
Reviewed-on: https://go-review.googlesource.com/c/go/+/261219


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