LU-6186 tests: avoid errors in using >&- bash close syntax

LU-6186 tests: avoid errors in using >&- bash close syntax

Seems that recent EL7 bash version include changes in handling
the ">&-" redirection/close syntax, which cause "Bad file
descriptor" error. Use >/dev/null to avoid this.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Iac3e54ef69f0c5c241b87a9e307e059974a4caf3
Reviewed-on: http://review.whamcloud.com/13857
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>