LU-8344 test: fix sanity 256

LU-8344 test: fix sanity 256

The test error
 Changelog catalog has wrong number of slots 1
is a result of the debugfs dump happaned before a previous
changes were commited to a disk.

The patch adds mds sync before debugfs command.
Also it fixes temp file removal.

Signed-off-by: Alexander Boyko <c17825@cray.com>
Change-Id: Ic14a58956642f419b0f6d695027f88a0cad9fd39
Test-Parameters: trivial testlist=sanity
Reviewed-on: https://review.whamcloud.com/29598
Reviewed-by: Sergey Cheremencev <cherementsev@gmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>