LU-2935 tests: Test for layout swap on open unlinked file

LU-2935 tests: Test for layout swap on open unlinked file

This regression test verifies that layout swap works correctly on an
open unlinked file

Signed-off-by: Swapnil Pimpale <spimpale@ddn.com>
Change-Id: Ib0de7de21286c3c9b37625008432ffd080de589b
Reviewed-on: http://review.whamcloud.com/7301
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>