cursor_valid_after_shard_stepdown.js can fail because of an unintended metadata refresh

When running the sharding_csrs_continuous_config_stepdown suite, the cursor_valid_after_shard_stepdown.js test expects to have stale sharding metadata, but the following scenario might happen:

1. The primary node steps down as part of the test
2. A step down happens on the config server
3. The new node is elected primary for the first time, so it runs the periodic index check aggregation
4. The sharding metadata is refreshed on the shard
5. The test assertion fails because it is expecting a stale sharding metadata

One way to solve this is to disable the periodic index check like this test