Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/PausableMock.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../security/Pausable.sol";
+
+contract PausableMock is Pausable {
+    bool public drasticMeasureTaken;
+    uint256 public count;
+
+    constructor() {
+        drasticMeasureTaken = false;
+        count = 0;
+    }
+
+    function normalProcess() external whenNotPaused {
+        count++;
+    }
+
+    function drasticMeasure() external whenPaused {
+        drasticMeasureTaken = true;
+    }
+
+    function pause() external {
+        _pause();
+    }
+
+    function unpause() external {
+        _unpause();
+    }
+}