Update `L2OutputOracle` invariant test

packages/contracts-bedrock/contracts/test/invariants/L2OutputOracle.t.sol

 pragma solidity 0.8.15;
 
 import { L2OutputOracle_Initializer } from "../CommonTest.t.sol";
+import { L2OutputOracle } from "../../L1/L2OutputOracle.sol";
+import { Vm } from "forge-std/Vm.sol";
+
+contract L2OutputOracle_Proposer {
+    L2OutputOracle internal oracle;
+    Vm internal vm;
+
+    constructor(L2OutputOracle _oracle, Vm _vm) {
+        oracle = _oracle;
+        vm = _vm;
+    }
+
+    /**
+     * @dev Allows the actor to propose an L2 output to the `L2OutputOracle`
+     */
+    function proposeL2Output(
+        bytes32 _outputRoot,
+        uint256 _l2BlockNumber,
+        bytes32 _l1BlockHash,
+        uint256 _l1BlockNumber
+    ) external {
+        // Act as the proposer and propose a new output.
+        vm.prank(oracle.PROPOSER());
+        oracle.proposeL2Output(_outputRoot, _l2BlockNumber, _l1BlockHash, _l1BlockNumber);
+    }
+}
 
 contract L2OutputOracle_MonotonicBlockNumIncrease_Invariant is L2OutputOracle_Initializer {
+    L2OutputOracle_Proposer internal actor;
+
     function setUp() public override {
         super.setUp();
 
-        // Set the target contract to the oracle proxy
-        targetContract(address(oracle));
-        // Set the target sender to the proposer
-        targetSender(address(proposer));
+        // Create a proposer actor.
+        actor = new L2OutputOracle_Proposer(oracle, vm);
+
+        // Set the target contract to the proposer actor.
+        targetContract(address(actor));
+
         // Set the target selector for `proposeL2Output`
         // `proposeL2Output` is the only function we care about, as it is the only function
         // that can modify the `l2Outputs` array in the oracle.
         bytes4[] memory selectors = new bytes4[](1);
-        selectors[0] = oracle.proposeL2Output.selector;
-        FuzzSelector memory selector = FuzzSelector({
-            addr: address(oracle),
-            selectors: selectors
-        });
+        selectors[0] = actor.proposeL2Output.selector;
+        FuzzSelector memory selector = FuzzSelector({ addr: address(actor), selectors: selectors });
         targetSelector(selector);
     }
 

packages/contracts-bedrock/invariant-docs/L2OutputOracle.md

 # `L2OutputOracle` Invariants
 
 ## The block number of the output root proposals should monotonically increase.
-**Test:** [`L2OutputOracle.t.sol#L32`](../contracts/test/invariants/L2OutputOracle.t.sol#L32)
+**Test:** [`L2OutputOracle.t.sol#L59`](../contracts/test/invariants/L2OutputOracle.t.sol#L59)
 
 When a new output is submitted, it should never be allowed to correspond to a block number that is less than the current output.