os,time: fix tests on iOS

When fixing tests for for self-hosted iOS builds, I
broke hosted builds.

Updates #31722

Change-Id: Id4e7d234fbd86cb2d29d320d75f4441efd663d12
Reviewed-on: https://go-review.googlesource.com/c/go/+/174698


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>