testing/synctest: remove Run

Run (experimental) is replaced by Test.

Fixes #74012

Change-Id: I1721e1edfbcb4f1fe2159dc0430a13685b2d08c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/709355
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: seankhliao