Freeze l1 height in the mock contract

contracts/src/LightClient.sol

@@ -377,9 +377,11 @@ contract LightClient is Initializable, OwnableUpgradeable, UUPSUpgradeable {
     /// L1 Block Number
     /// @param blockNumber The L1 block number
     /// @param threshold The number of blocks updates to this contract is allowed to lag behind
+    /// Marked as `virtual` for easily testing.
     function lagOverEscapeHatchThreshold(uint256 blockNumber, uint256 threshold)
         public
         view
+        virtual
         returns (bool)
     {
         uint256 updatesCount = stateUpdateBlockNumbers.length;

contracts/test/mocks/LightClientMock.sol

@@ -10,6 +10,9 @@ import { LightClientStateUpdateVKMock as VkLib } from "./LightClientStateUpdateV
 
 /// @dev A helper that wraps LightClient contract for testing
 contract LightClientMock is LC {
+    bool internal hotShotDown;
+    uint256 internal frozenL1Height;
+
     constructor(LC.LightClientState memory genesis, uint32 numBlockPerEpoch) LC() {
         _initializeState(genesis, numBlockPerEpoch);
     }
@@ -65,4 +68,26 @@ contract LightClientMock is LC {
             hotShotCommitments.push(values[i]);
         }
     }
+
+    function setHotShotDownSince(uint256 l1Height) public {
+        hotShotDown = true;
+        frozenL1Height = l1Height;
+    }
+
+    function setHotShotUp() public {
+        hotShotDown = false;
+    }
+
+    /// @dev override the production-implementation with frozen data
+    function lagOverEscapeHatchThreshold(uint256 blockNumber, uint256 threshold)
+        public
+        view
+        override
+        returns (bool)
+    {
+        if (hotShotDown) {
+            return blockNumber - frozenL1Height > threshold;
+        }
+        return super.lagOverEscapeHatchThreshold(blockNumber, threshold);
+    }
 }