test/objectstore: set pool for fsck test

test/objectstore: set pool for fsck test

since 0bd2546eaca72ed0122a9c2648df4bef05b0d5d2, we check the pool id
of object when performing fsck to ensure we are looking at the right
collection, but the test is still using the pool id set by the
constructor of hobject_t. so all objects we created in that test belong
to the POOL_META. while the collection is created with the pool id of
`555`. hence the test fails.

Fixes: http://tracker.ceph.com/issues/36099
Signed-off-by: Kefu Chai <kchai@redhat.com>