Claim reward bug test (#924)

## Describe your changes
Claim reward bug test.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Tests**
- Enhanced test coverage for the scenario where rewards have already
been claimed, ensuring robustness.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

pallets/ocex/src/tests.rs

@@ -2468,6 +2468,7 @@ fn test_do_claim_lmp_rewards_happy_path() {
 		assert_eq!(Balances::free_balance(&main_account), 999999999900u128);
 		assert_ok!(OCEX::do_claim_lmp_rewards(main_account.clone(), epoch, trading_pair));
 		assert_eq!(Balances::free_balance(&main_account), 200999999999900u128);
+		assert_noop!(OCEX::do_claim_lmp_rewards(main_account.clone(), epoch, trading_pair), Error::<Test>::RewardAlreadyClaimed);
 	})
 }