LU-8435 tests: slab alloc error does not LBUG

LU-8435 tests: slab alloc error does not LBUG

Under memory pressure, for instance using a memory cgroup
and kmem.limit_in_bytes enforced (SLURM does this),
osc_extent_alloc() could fail and error handling will
hit an LBUG.

Add a test for this.

Test-Parameters: trivial testlist=sanity,sanity,sanity

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Change-Id: I135f05ee4be14521522c949e50bd4c8deb1f099a
Reviewed-on: https://review.whamcloud.com/21745
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>