Allow logger tests to run in parallel

Both variants of logger_test (dedupe/rotate) use the same
filenames. This means that if they are in parallel one will likely
fail.

Update them to each use a unique file prefix.

Change-Id: Ia8cb50a4f073f4b1ed0367863ee1ee55e446bb4e
Reviewed-on: http://review.couchbase.org/42521
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>