ceph_test_rados_api_tier: disable LibRadosTierECPP::HitSetWrite

Disable this test until hitget-get reliably works on EC pools (currently
it does not, and this test usually passes only because we get the in-memory
HitSet).

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1971 from ceph/wip-ec-hitset

ceph_test_rados_api_tier: disable LibRadosTierECPP::HitSetWrite

Reviewed-by: Loic Dachary <loic@dachary.org>