testing: restore os.Exit(0) after every call to (*M).Run
cmd/go.TestScript/test_main_twice demonstrates a program that invokes
(*M).Run twice in a row. If we only restore os.Exit(0) in m.afterOnce,
we will fail to restore it after the second run and fail the test
process despite both runs passing.
Updates #29062
Updates #23129
Change-Id: Id22ec68f1708e4583c8dda14a8ba0efae7178b85
Reviewed-on: https://go-review.googlesource.com/c/go/+/251262
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>