LU-11403 tests: Fix $tfile usage

LU-11403 tests: Fix $tfile usage

We cannot just use raw $tfile - we must use something under
$DIR.  This is resulting in failures because $tfile exists.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Iea6356cabb1623606bf926ce80c55a3210c0b535
Reviewed-on: https://review.whamcloud.com/34698
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>