LU-4283 test: auster "-s" flag does not run SLOW tests

LU-4283 test: auster "-s" flag does not run SLOW tests

Using the "-s" flag when calling auster is supposed to
run tests that are marked as "SLOW". The SLOW variable was
not exported and, thus, was not recognized as being set
in the test scripts. SLOW is now exported.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I52898d183163392144d1e809df35ab6278f06df2
Reviewed-on: http://review.whamcloud.com/8351
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>