bump lookahead to 3 for testnet genesis (#7252)

This is the right value after
https://github.com/paritytech/polkadot-sdk/pull/4880, which corresponds
to an allowedAncestryLen of 2 (which is the default)

WIll fix https://github.com/paritytech/polkadot-sdk/issues/7105

polkadot/runtime/rococo/src/genesis_config_presets.rs

@@ -134,7 +134,7 @@ fn default_parachains_host_configuration(
 				1u8 << (FeatureIndex::CandidateReceiptV2 as usize),
 		),
 		scheduler_params: SchedulerParams {
-			lookahead: 2,
+			lookahead: 3,
 			group_rotation_frequency: 20,
 			paras_availability_period: 4,
 			..Default::default()

polkadot/runtime/westend/src/genesis_config_presets.rs

@@ -137,7 +137,7 @@ fn default_parachains_host_configuration(
 				1u8 << (FeatureIndex::CandidateReceiptV2 as usize),
 		),
 		scheduler_params: SchedulerParams {
-			lookahead: 2,
+			lookahead: 3,
 			group_rotation_frequency: 20,
 			paras_availability_period: 4,
 			..Default::default()

polkadot/zombienet-sdk-tests/tests/elastic_scaling/doesnt_break_parachains.rs

@@ -120,8 +120,8 @@ async fn doesnt_break_parachains_test() -> Result<(), anyhow::Error> {
 	assert_eq!(
 		cq,
 		[
-			(CoreIndex(0), [para_id, para_id].into_iter().collect()),
-			(CoreIndex(1), [para_id, para_id].into_iter().collect()),
+			(CoreIndex(0), std::iter::repeat(para_id).take(3).collect()),
+			(CoreIndex(1), std::iter::repeat(para_id).take(3).collect()),
 		]
 		.into_iter()
 		.collect()