sync: hide test of misuse of Cond from vet

The test wants to check that copies of Cond are detected at runtime.
Make a copy that isn't detected by vet at compile time.

Change-Id: I933ab1003585f75ba96723563107f1ba8126cb72
Reviewed-on: https://go-review.googlesource.com/108557


Reviewed-by: Ian Lance Taylor <iant@golang.org>