Replace Arc.clone()s with Arc::clone() in BP tests

`arc.clone()` leaves the code somewhat ambiguous to whether we're
doing an expensive deep-clone operation or if we're doing a
(relatively) cheap pointer-copy-and-atomic-increment operation.

More importantly, it leaves entirely unclear what the semantics of
the object we just created are - does updating it update the
original or leave it be?

Thus, here, we replace `arc.clone()` calls with `Arc::clone()` in
`lightning-background-processor` tests.

Author: TheBlueMatt