LU-7149 tests: restore writethrough_cache_enable

Test sanity.sh test_224c is failed as expected if executed separately
and passes if executed by automatic system. Tests 155d,155f,155h,156
do "set_cache writethrough off" and don't restore the state. This
makes next tests work incorrectly.

This patch adds writethrough_cache_enable restore for each function
above.

Test-Parameters: trivial

Signed-off-by: Artem Blagodarenko <artem.blagodarenko@seagate.com>
Xyratex-bug-id: MRP-2590
Change-Id: I5f4f3f6c419a3aa415426607e776403da9822c2c
Reviewed-on: http://review.whamcloud.com/16424
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>