LU-3852 tests: skip tests with large file when no room

LU-3852 tests: skip tests with large file when no room

Strengthen sanity-hsm sub-tests using large files by skipping
then if not enough space instead to fail.

Also re-enable test_251, and test_[200,221,223b] (for LU-4178),
who have all been wrongly disabled because of their multiple
failures on an auto-test node with very low disk space for the
Servers VMs!!

Also fix a bug in sanity/test_56w that should not be run if
$OSTCOUNT < 2, that was triggered due to my very restictive
test environment osscount/ostcount=1 to limit the Lustre FS size
as much as possible.

Test-Parameters: ostcount=1 ostsizegb=1 envdefinitions=SLOW=yes
Test-Parameters: osscount=1 mdscount=1 testlist=sanity-hsm
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I26bc36e4304a916f29149e6ad19306ed59c9a616
Reviewed-on: http://review.whamcloud.com/12456
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>