Add Echidna test skeleton

test/echidna/EchidnaUtils.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.19;
+
+library EchidnaUtils {
+    event AssertionFailed(string msg, uint256 expected, uint256 actualValue);
+
+    function between(uint256 random, uint256 low, uint256 high) internal pure returns (uint256) {
+        return low + random % (high - low);
+    }
+}

test/echidna/PairEchidna.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.19;
+
+import "./EchidnaUtils.sol";
+
+contract PairEchidna {
+    bool isSetup;
+
+    function setup() private {
+        isSetup = true;
+    }
+
+    function echidnaTestTodo() public {
+        //************************* Pre-Conditions *************************/
+        if (!isSetup) setup();
+
+        //************************* Action *************************/
+
+        //************************* Post-Conditions *************************/
+    }
+}

test/echidna/config.yaml

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+testMode: assertion
+corpusDir: corpus
+testLimit: 50000