tests: perf_fast_forward: Fix live_range not being initialized
tests: perf_fast_forward: Fix live_range not being initialized

Broken in 470552b7ab0d5feb79b7e65bc71d63197ded5c82

Causes test failure when running with --cache-enabled
Message-Id: <cc4afe65d5dd1fcc9ac6510e5203e0b8d44b44e8@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla