Use temporary filename for configcache tests

If tests are run in parallel, there may be a race condition between two
instances.

Change-Id: I8f1e452588fd27f57ec2a40912f2907a42ea52d0
Reviewed-on: http://review.couchbase.org/29611
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>