[dev.typeparams] test: update all the typeparam tests to use the new union/tilde syntax

Did a mix of tilde and non-tilde usage. Tilde notation is not quite
fully functional, so no tests are currently trying to distinguish
(fail/not fail) based on tilde usage.

Change-Id: Ib50cec2fc0684f9d9f3561c889fd44c7a7af458c
Reviewed-on: https://go-review.googlesource.com/c/go/+/324572


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