src/test: allow ENOENT if target object of tier_flush has snapshots

src/test: allow ENOENT if target object of tier_flush has snapshots

Tier_flush can return ENOENT if the recovery procedure for
adjacent clones are in progress. Allowing ENOENT might not
cover some cases, but this has some advantages:
1) tier_flush is a write operation that rarely returns ENOENT
2) other operations in combination test can cover ENOENT case,
minimizing OSD changes.

Signed-off-by: Myoungwon Oh <myoungwon.oh@samsung.com>