crypto/x509: save the temp dir in TestReadUniqueDirectoryEntries

In CL 231958, TempDir was changed to create a new temp directory on
each allocation, on the theory that it is easy to save in a variable
for callers that want the same directory repeatedly. Apply that
transformation here.

Updates #38850

Change-Id: Ibb014095426c33038e0a2c95303579cf95d5c3ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/234582


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>