store_test.cc: fix unchecked return value

CID 1188126 (#1 of 1): Unchecked return value (CHECKED_RETURN)
 2. check_return: Calling function "ObjectStore::stat(coll_t,
    ghobject_t const &, stat *, bool)" without checking return value
    (as is done elsewhere 8 out of 9 times).
 3. unchecked_value: No check of the return value of "this->store->stat(
    coll_t(this->cid), hoid, &buf, false)".

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>