Resolve linting warnings for `ValidatorRewarder ` tests

[dtbuchholz]

See https://github.com/hokunet/contracts/actions/runs/12753927732/job/35546622290#step:7:12. Logs here:

Warning (2072): Unused local variable.
   --> test/ValidatorRewarder.t.sol:375:9:
    |
375 |         uint256 baseReward = 1547[12](https://github.com/hokunet/contracts/actions/runs/12753927732/job/35546622290#step:7:13)667492044;
    |         ^^^^^^^^^^^^^^^^^^

Warning (2072): Unused local variable.
  --> test/ValidatorRewarderFFI.t.sol:55:[13](https://github.com/hokunet/contracts/actions/runs/12753927732/job/35546622290#step:7:14):
   |
55 |             uint256 expectedSupplyDelta = vm.parseJsonUint(jsonStr, ".supply_delta");
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Warning (2018): Function state mutability can be restricted to view
  --> test/ValidatorRewarder.t.sol:80:5:
   |
80 |     function testInitialState() public {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning (2018): Function state mutability can be restricted to view
   --> test/ValidatorRewarder.t.sol:116:5:
    |
116 |     function testInitialSubnetSetup() public {
    |     ^ (Relevant source part starts here and spans across multiple lines).

Warning (2018): Function state mutability can be restricted to view
   --> test/ValidatorRewarder.t.sol:[15](https://github.com/hokunet/contracts/actions/runs/12753927732/job/35546622290#step:7:16)2:5:
    |
152 |     function testInitialTokenSetup() public {
    |     ^ (Relevant source part starts here and spans across multiple lines).