Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/crosschain/receivers.sol

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+import "../../access/Ownable.sol";
+import "../../crosschain/amb/CrossChainEnabledAMB.sol";
+import "../../crosschain/arbitrum/CrossChainEnabledArbitrumL1.sol";
+import "../../crosschain/arbitrum/CrossChainEnabledArbitrumL2.sol";
+import "../../crosschain/optimism/CrossChainEnabledOptimism.sol";
+import "../../crosschain/polygon/CrossChainEnabledPolygonChild.sol";
+
+abstract contract Receiver is CrossChainEnabled {
+    // we don't use Ownable because it messes up testing for the upgradeable contracts
+    /// @custom:oz-upgrades-unsafe-allow state-variable-immutable state-variable-assignment
+    address public immutable owner = msg.sender;
+
+    function crossChainRestricted() external onlyCrossChain {}
+
+    function crossChainOwnerRestricted() external onlyCrossChainSender(owner) {}
+}
+
+/**
+ * AMB
+ */
+contract CrossChainEnabledAMBMock is Receiver, CrossChainEnabledAMB {
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address bridge) CrossChainEnabledAMB(bridge) {}
+}
+
+/**
+ * Arbitrum
+ */
+contract CrossChainEnabledArbitrumL1Mock is Receiver, CrossChainEnabledArbitrumL1 {
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address bridge) CrossChainEnabledArbitrumL1(bridge) {}
+}
+
+contract CrossChainEnabledArbitrumL2Mock is Receiver, CrossChainEnabledArbitrumL2 {}
+
+/**
+ * Optimism
+ */
+contract CrossChainEnabledOptimismMock is Receiver, CrossChainEnabledOptimism {
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address bridge) CrossChainEnabledOptimism(bridge) {}
+}
+
+/**
+ * Polygon
+ */
+contract CrossChainEnabledPolygonChildMock is Receiver, CrossChainEnabledPolygonChild {
+    /// @custom:oz-upgrades-unsafe-allow constructor
+    constructor(address bridge) CrossChainEnabledPolygonChild(bridge) {}
+}