Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/token/ERC1155ReceiverMock.sol

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../token/ERC1155/IERC1155Receiver.sol";
+import "../../utils/introspection/ERC165.sol";
+
+contract ERC1155ReceiverMock is ERC165, IERC1155Receiver {
+    bytes4 private _recRetval;
+    bool private _recReverts;
+    bytes4 private _batRetval;
+    bool private _batReverts;
+
+    event Received(address operator, address from, uint256 id, uint256 value, bytes data, uint256 gas);
+    event BatchReceived(address operator, address from, uint256[] ids, uint256[] values, bytes data, uint256 gas);
+
+    constructor(bytes4 recRetval, bool recReverts, bytes4 batRetval, bool batReverts) {
+        _recRetval = recRetval;
+        _recReverts = recReverts;
+        _batRetval = batRetval;
+        _batReverts = batReverts;
+    }
+
+    function onERC1155Received(
+        address operator,
+        address from,
+        uint256 id,
+        uint256 value,
+        bytes calldata data
+    ) external override returns (bytes4) {
+        require(!_recReverts, "ERC1155ReceiverMock: reverting on receive");
+        emit Received(operator, from, id, value, data, gasleft());
+        return _recRetval;
+    }
+
+    function onERC1155BatchReceived(
+        address operator,
+        address from,
+        uint256[] calldata ids,
+        uint256[] calldata values,
+        bytes calldata data
+    ) external override returns (bytes4) {
+        require(!_batReverts, "ERC1155ReceiverMock: reverting on batch receive");
+        emit BatchReceived(operator, from, ids, values, data, gasleft());
+        return _batRetval;
+    }
+}