block: test-iosched: Sleep before each test

In order to be sure that the packing statistics collected after the test
reflect *only* requests issued by the test (and not real request from
FS) - sleep before each test in order to give an already dispatched
requests time to complete.

Change-Id: If2f40efad1d79084a8ea85afe93cce58e49ff698
CRs-Fixed: 453712
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>