io/ioutil: make TestTempFile more robust

io/ioutil: make TestTempFile more robust

The first part of this test tries to confirm that we can't create
a TempFile in a non-existent directory, but does not ensure that
the non-existent directory really does not exist.  Instead, let's
create an empty temp directory, and use a non-existent subdir of
that.

Change-Id: I176f14ed5f5a2d7a8c29d8f6949755db69d7dbb6
Reviewed-on: https://go-review.googlesource.com/40914
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>