LU-7445 test: combine dependent sanity 51b and 51ba tests

LU-7445 test: combine dependent sanity 51b and 51ba tests

test_51ba() depends on test_51b() and skipped if run separately

Patch combines test_51b() and test_51ba() into test_51b().
$tdir is used for test dir name.

Test-Parameters: envdefinitions=ONLY=51b testlist=sanity
Seagate-bug-id: MRP-2290
Signed-off-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>
Change-Id: I6ee019a03d398e36c7007973614375c9ca22e349
Reviewed-on: http://review.whamcloud.com/17239
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>