LU-4030 tests: use free_fd() to allocate file descriptor

LU-4030 tests: use free_fd() to allocate file descriptor

free_fd() lists /proc/self/fd to find the smallest unused file
descriptor
sanity test_31n is changed to use free_fd() instead of using hardcoded
173
sanity test_236 is changed to use free_fd() instead of using "{FD}<>"
which is not available on eariler bash

Signed-off-by: Vladimir Saveliev <vladimir_saveliev@xyratex.com>
Change-Id: Ie13da99e4a758d7169454c7a6b3b58c594b30ecc
Reviewed-on: http://review.whamcloud.com/8181
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>