LU-7072 tests: Fix and re-enable test 78 in sanityn

LU-7072 tests: Fix and re-enable test 78 in sanityn

Early NRS policy tunings can fail for the following reasons:
  - Policy not started (ENODEV)
  - Policy not fully initialized (EAGAIN)

Make test_78 of sanityn handle both properly and re-enable it.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I1d68ad709d0ac1c8ee9dabebf565c786d7c904da
Reviewed-on: http://review.whamcloud.com/16178
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Andreas Dilger <andreas.dilger@intel.com>