contracts-periphery: update version on test contracts

Makes the contracts more lenient in the version
of solc that they can compile with, makes it easier
to begin porting the hardhat based tests to `foundry`.

packages/contracts-periphery/contracts/testing/helpers/CallRecorder.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 contract CallRecorder {
     struct CallInfo {

packages/contracts-periphery/contracts/testing/helpers/ExternalContractCompiler.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 import { ProxyAdmin } from "@eth-optimism/contracts-bedrock/contracts/universal/ProxyAdmin.sol";
 import { Proxy } from "@eth-optimism/contracts-bedrock/contracts/universal/Proxy.sol";

packages/contracts-periphery/contracts/testing/helpers/FailingReceiver.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 contract FailingReceiver {
     receive() external payable {

packages/contracts-periphery/contracts/testing/helpers/Reverter.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 contract Reverter {
     function doRevert() public pure {

packages/contracts-periphery/contracts/testing/helpers/SimpleStorage.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 contract SimpleStorage {
     mapping(bytes32 => bytes32) public db;

packages/contracts-periphery/contracts/testing/helpers/TestERC20.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 import { ERC20 } from "@rari-capital/solmate/src/tokens/ERC20.sol";
 

packages/contracts-periphery/contracts/testing/helpers/TestERC721.sol

 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.15;
+pragma solidity ^0.8.0;
 
 import { ERC721 } from "@rari-capital/solmate/src/tokens/ERC721.sol";