LU-15011 tests: pool spill test modifications

LU-15011 tests: pool spill test modifications

Make the following modifications to the ost-pools
test suite:
test 29 - change check for 'when striping is specified
          explicitly' file from 'file-2' to 'file-3'

test 30 - Add bad parameter check for setting the threshold
          below zero

test 31 - 'do_nodes $mdts $LCTL get_param lod.*.pool.*'
          doesn’t print anything. Change to
          'do_nodes $mdts $LCTL get_param lod.*.pool.*.spill*'

Fixes: 0a998f4723 (“LU-14825 lod: pool spilling”)
Test-Parameters: trivial testlist=ost-pools
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: Icbdc3d42b7f7609bc57cc37830975d831125d659
Reviewed-on: https://review.whamcloud.com/45056
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>