EX-3442 tests: use wait_file_resync in hot-pools test 15

EX-3442 tests: use wait_file_resync in hot-pools test 15

This patch replaces "$LFS mirror resync" with
"wait_file_resync" in hot-pools test 15 to avoid
racing with lamigo's "$LFS mirror resync".

Test-Parameters: trivial testlist=hot-pools,hot-pools

Change-Id: I48ffb7d6a33b664359f227d1f693369feffa70b6
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47233
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>