test_runners: increase some timeouts to make tests less flaky (#5521)

## Problem
- `test_heavy_write_workload` is flaky, and fails because of to
statement timeout
- `test_wal_lagging` is flaky and fails because of the default pytest
timeout (see https://github.com/neondatabase/neon/issues/5305)

## Summary of changes
- `test_heavy_write_workload`: increase statement timeout to 5 minutes
(from default 2 minutes)
- `test_wal_lagging`: increase pytest timeout to 600s (from default
300s)

test_runner/performance/test_wal_backpressure.py

@@ -65,7 +65,7 @@ def start_heavy_write_workload(env: PgCompare, n_tables: int, scale: int, num_it
 
     def start_single_table_workload(table_id: int):
         for _ in range(num_iters):
-            with env.pg.connect().cursor() as cur:
+            with env.pg.connect(options="-cstatement_timeout=300s").cursor() as cur:
                 cur.execute(
                     f"INSERT INTO t{table_id} SELECT FROM generate_series(1,{new_rows_each_update})"
                 )