testing: remove call to os.Exit in documentation for TestMain
In the example for the TestMain function, os.Exit was called explicitly,
which is no longer necessary since Go 1.15 (see change #219639).
Updates #34129
Change-Id: Ia8021de0d5699742adbb7ab6d28b11c841e596dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/596977 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>