LU-797 tests: improve test_23b of ost-pools.sh

LU-797 tests: improve test_23b of ost-pools.sh

- fail the test once the written space is greater than the
  available space.
- use it's own maxfree limit, and increase the limit a lot.
- terminate the test when the write speed is too slow.

Test-Parameters: envdefinitions=SLOW=yes testlist=ost-pools
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Ib7deca89ebd7267fa7be0a51ceac01fe3cb2df65
Reviewed-on: http://review.whamcloud.com/4474
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Hudson