Add a constant query function to FaultDisputeGameN, modify the attack branch variable to uint256, to unify with other variable types.

[JustXxx]

forge test --mc FaultDisputeGameN_Test

...
[PASS] test_stepAttackDummyClaim_attackBranch1_succeeds() (gas: 1008885)
[PASS] test_stepAttackDummyClaim_attackBranch2_succeeds() (gas: 1008850)
[PASS] test_stepAttackDummyClaim_attackBranch3_succeeds() (gas: 1006917)
[PASS] test_stepAttackDummyClaim_attackDACalldata_succeeds() (gas: 1027855)
Suite result: ok. 71 passed; 0 failed; 0 skipped; finished in 605.46ms (1.81s CPU time)

Ran 1 test suite in 627.54ms (605.46ms CPU time): 71 tests passed, 0 failed, 0 skipped (71 total tests)

forge test --mc FaultDisputeN_1v1_Actors_Test

...
[PASS] test_static_1v1honestRootGenesisAbsolutePrestate_succeeds() (gas: 147880537)
[PASS] test_static_1v1honestRoot_succeeds() (gas: 159001375)
Suite result: ok. 11 passed; 0 failed; 0 skipped; finished in 7.35s (9.85s CPU time)

Ran 1 test suite in 7.37s (7.35s CPU time): 11 tests passed, 0 failed, 0 skipped (11 total tests)

[dajuguan]

Why attack branch variable's type need to be unify with other variable types?