tests: ceph-erasure-code-corpus must test SIMD variants

tests: ceph-erasure-code-corpus must test SIMD variants

ceph_erasure_code.cc and ceph_erasure_code_benchmark.cc failed to load
the plugins. It went unnoticed when
660ae5bcbb250b06cf88ec7f9a3f37b05c6c8118 was reviewed because

  * ceph_erasure_code_benchmark is not used in make check
  * qa/workunits/erasure-code/encode-decode-non-regression.sh silently
    interpreted the failure as the absence of SIMD variants

http://tracker.ceph.com/issues/12933 Fixes: #12933

Signed-off-by: Loic Dachary <ldachary@redhat.com>