Avoid the use of constant absolute temp files in tests.  This could
produce flaky results, for example on multiuser development machines.

Change-Id: Ia76157a0660fbe294bb31a46ded886cea5deec97
Reviewed-on: https://go-review.googlesource.com/40916Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>