fix(get_db_tables): iterate over copy of tables keys
fix(get_db_tables): iterate over copy of tables keys

while iterating over session.cluster.metadata.keyspaces[ks].tables.keys()
we can get into situation when new table is added or the structure
is altered by the driver in background, hence we need to iterate
over the copy of the keys otherwize we can get
'RuntimeError: dictionary changed size during iteration'

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests