ceph_test_rados_api_tier: add EvictSnap2 test case

Verify an evict doesn't create a snapdir object.  Reproduces #8629

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 398c74eacb1ce4e573aef0d24718a5925d90272b)
osd/ReplicatedPG: do not clone or preserve snapdir on cache_evict

If we cache_evict a head in a cache pool, we need to prevent
make_writeable() from cloning the head and finish_ctx() from
preserving the snapdir object.

Fixes: #8629
Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit ce8eefca13008a9cce3aedd67b11537145e1fd77)
Merge pull request #2737 from ceph/wip-9629-firefly

osd: do not clone/preserve snapdir on cache-evict (firefly backport)

Reviewed-by: Samuel Just <sjust@redhat.com>