LU-2918 tests: sanity 184c to start copy before layout swap

LU-2918 tests: sanity 184c to start copy before layout swap

The test script has to make sure dd already starts to copy file
before swapping layout, otherwise the attempt to open file1 would
fail.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I8217c9a38f1d09830c0ab259f65c9716c06736d1
Reviewed-on: http://review.whamcloud.com/5617
Tested-by: Hudson
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>