tests/performance: reconcile until idle before benchmark (#11435)

We'd like to run benchmarks starting from a steady state. To this end,
do a reconciliation round before proceeding with the benchmark.

This is useful for benchmarks that use tenant dir snapshots since a
non-standard tenant configuration is used to generate the snapshot. The
storage controller is not aware of the non default tenant configuration
and will reconcile while the bench is running.

test_runner/performance/pageserver/util.py

@@ -40,6 +40,8 @@ def ensure_pageserver_ready_for_benchmarking(env: NeonEnv, n_tenants: int):
             for layer in info.historic_layers:
                 assert not layer.remote
 
+    env.storage_controller.reconcile_until_idle(timeout_secs=60)
+
     log.info("ready")