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>
(cherry picked from commit c5b5ed672c039cc4fa04772e065a59d3b1df803a)