testing/synctest: remove Run

Run (experimental) is replaced by Test.

Fixes golang#74012

Change-Id: I1721e1edfbcb4f1fe2159dc0430a13685b2d08c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/709355
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: seankhliao