Otherwise, the test cannot create new files in the directory.

Updates #32407
Updates #30316

Change-Id: Ief0df94a202be92f57d458d4ab4e4daa9ec189b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/206458
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>