bench: stop immediatedly in some benches (#8713)

It seems that some benchmarks are failing because they are simply not
stopping to ingest wal on shutdown. It might mean that the tests were
never ran on a stable pageserver situation and WAL has always been left
to be ingested on safekeepers, but let's see if this silences the
failures and "stops the bleeding".

Cc: https://github.com/neondatabase/neon/issues/8712

test_runner/performance/pageserver/pagebench/test_ondemand_download_churn.py

@@ -62,6 +62,9 @@ def test_download_churn(
 
     run_benchmark(env, pg_bin, record, io_engine, concurrency_per_target, duration)
 
+    # see https://github.com/neondatabase/neon/issues/8712
+    env.stop(immediate=True)
+
 
 def setup_env(neon_env_builder: NeonEnvBuilder, pg_bin: PgBin):
     remote_storage_kind = s3_storage()

test_runner/performance/test_layer_map.py

@@ -36,3 +36,6 @@ def test_layer_map(neon_env_builder: NeonEnvBuilder, zenbenchmark):
     with zenbenchmark.record_duration("test_query"):
         cur.execute("SELECT count(*) from t")
         assert cur.fetchone() == (n_iters * n_records,)
+
+    # see https://github.com/neondatabase/neon/issues/8712
+    env.stop(immediate=True)