LU-11233 tests: fix gcc8 build warnings

LU-11233 tests: fix gcc8 build warnings

this patch covers Lustre tests

Lustre-change: https://review.whamcloud.com/34661
Lustre-commit: 6733fbff9a682bcec5fdca6f7062c24f0fe27cfe

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I6345d603772fb32bbc4b38a758a3e97f0361d116
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35005
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>