Disable fsync in arbitrary config tests (#5800)

We have fsync enabled for regular tests already in `pg_regress_multi.pl`.
This does the same for the arbitrary config tests.

On my machine this changes the runtime from the following command from
~37 to ~25 seconds:

```bash
make -C src/test/regress/ check-arbitrary-configs CONFIGS=CitusDefaultClusterConfig
```

src/test/regress/citus_tests/config.py

@@ -103,6 +103,7 @@ class CitusBaseClusterConfig(object, metaclass=NewInitCaller):
         self.settings = {
             "shared_preload_libraries": "citus",
             "log_error_verbosity": "terse",
+            "fsync": False,
             "citus.node_conninfo": "sslmode=prefer",
             "citus.enable_repartition_joins": True,
             "citus.repartition_join_bucket_count_per_node": 2,