os: give race detector chance to override Exit(0)

os: give race detector chance to override Exit(0)

Racy tests do not fail currently, they do os.Exit(0).
So if you run go test without -v, you won't even notice.
This was probably introduced with testing.TestMain.

Racy programs do not have the right to finish successfully.

Change-Id: Id133d7424f03d90d438bc3478528683dd02b8846
Reviewed-on: https://go-review.googlesource.com/4371
Reviewed-by: Russ Cox <rsc@golang.org>