testing: fail Example tests that invoke runtime.Goexit

Previously, if an example test invoked runtime.Goexit, it would
pass yet hang until a timeout, while regular tests that invoke
runtime.Goexit do fail. This change removes that inconsistent
behavior and makes such example tests fail, and panic with an
indication of having invoked runtime.Goexit.

Fixes #41084

Change-Id: I0ffa152204f2b1580f4d5d6961ba1ce6b13fc022
Reviewed-on: https://go-review.googlesource.com/c/go/+/251857


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>