fix the unit test of bond

[JustXxx]

forge test --mc FaultDisputeGameN_Test --mt .[bB]ond.

Ran 7 tests for test/dispute/FaultDisputeGameN.t.sol:FaultDisputeGameN_Test
[PASS] testFuzz_challengeRootL2Block_receivesBond_succeeds(bytes32,bytes32,uint256) (runs: 65, μ: 907156, ~: 907156)
[PASS] test_getRequiredBond_outOfBounds_reverts() (gas: 12465)
[PASS] test_getRequiredBond_succeeds() (gas: 36816)
[PASS] test_move_incorrectBondAmount_reverts() (gas: 33335)
[PASS] test_resolve_bondPayoutsSeveralActors_succeeds() (gas: 1407849)
[PASS] test_resolve_bondPayouts_succeeds() (gas: 1401801)
[PASS] test_resolve_leftmostBondPayout_succeeds() (gas: 1596079)
Suite result: ok. 7 passed; 0 failed; 0 skipped; finished in 473.35ms (178.32ms CPU time)

forge test --mc FaultDisputeGameN_Test --mt test_claimCredit_claimAlreadyResolved_reverts

Ran 1 test for test/dispute/FaultDisputeGameN.t.sol:FaultDisputeGameN_Test
[PASS] test_claimCredit_claimAlreadyResolved_reverts() (gas: 835658)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 327.36ms (874.00µs CPU time)

[dajuguan]

All tests passed. LGTM