We achieve this by always running all tests that create files in a
fresh temporary directory, rather than just on darwin/{arm,arm64}.
As a bonus, this lets us simplify the cleanup code for these tests
and assume their working directory starts out empty.

Updates #28387

Change-Id: I952007ae390a2451c9a368da26c7f9f5af64b2ba
Reviewed-on: https://go-review.googlesource.com/c/145283Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>