LU-4329 tests: initialize $PREV_RESULT in wait_update()

LU-4329 tests: initialize $PREV_RESULT in wait_update()

Initialize variable PREV_RESULT in wait_update() to avoid useless
output like "Changed after 0s: from '' to '1'".

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I2aea8547892a36e92e8d0c54ac541397a102e76c
Reviewed-on: http://review.whamcloud.com/8449
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>