LU-10835 tests: unload dm-flakey module

LU-10835 tests: unload dm-flakey module

In test-framework.sh, dm-flakey module is loaded in dm_flakey_supported()
but it's not unloaded anywhere. This patch unloads it in dm_cleanup_dev().

Test-Parameters: trivial

Change-Id: Ie40b8bc1a36a7a271cb333b81965fbe136268db1
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/31739
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>