LU-2805 tests: compare the file correctly for layout swapping

LU-2805 tests: compare the file correctly for layout swapping

The size of file ref2 may be less than copied bytes so the result of
command cmp will be false due to EOF. Check differ string instead.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I344f90bf4b75f535961f1acdc5574aeca8e85126
Reviewed-on: http://review.whamcloud.com/5420
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>