LU-16168 tests: fix get_slave_nr in sanity-quota

LU-16168 tests: fix get_slave_nr in sanity-quota

Redirect output of wait_update_facet in get_slave_nr
to /dev/null/. Otherwise if wait_update_cond prints
something while waiting, get_slave_nr returns this
output together with slave number causing test_68
to fail.

Test-Parameters: trivial testlist=sanity-quota
Test-Parameters: testlist=sanity-quota fstype=zfs
Fixes: 83dd308db5 ("LU-15460 test: wait quota pool to be prepared")
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: I813ed31db864897372c7eb6aab4e1f5a4b955f49
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50659
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>