verify that re-using ARFCN+BSIC pairs across separate BSS works
A remote BSS may re-use an ARFCN+BSIC pair that is also used in the local BSS.
Verify in a test that an inter-BSC handover to the remote cell indeed works, and does not end up in the local-BSS cell that has the same ARFCN+BSIC but is not a direct neighbor for the handover source cell.
In handover_start(), it looks like bts_by_neighbor_ident() would prefer to pick a local-BSS cell with the same ARFCN+BSIC of the remote cell,
and should instead invoke an inter-BSC handover when only a remote cell with that ARFCN+BSIC is listed as explicit neighbor for the given source BTS.