tests: spurious failure fix for bug-1164613.t

Snapshot bricks are started asynchronously.
It might take sometime before accessing the file.


Change-Id: I62633974eca21f86632f7005ce6fd5c856b8be90
BUG: 1163543
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/9142
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>