test: add mock USTB subscription/redemption contract

Author: yan-man

src/test/mocks/MockUSTBSubscriptionRedemption.sol

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+pragma solidity ^0.8.10;
+
+import {IERC20} from '@openzeppelin/contracts/token/ERC20/IERC20.sol';
+import {SafeERC20} from '@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol';
+
+/**
+ * @title MockUSTBSubscription
+ */
+contract MockUSTBSubscription {
+  using SafeERC20 for IERC20;
+
+  address public immutable asset;
+  address public immutable liquidity;
+  uint256 public USTBPrice;
+
+  /**
+   * @param _asset Address of asset token, ie USTB
+   * @param _liquidity Address of liquidity token, ie USDC
+   */
+  constructor(address _asset, address _liquidity, uint256 _price) {
+    asset = _asset;
+    liquidity = _liquidity;
+    setUSTBPrice(_price);
+  }
+
+  function test_coverage_ignore() public virtual {
+    // Intentionally left blank.
+    // Excludes contract from coverage.
+  }
+
+  /**
+   * @notice Subscribe to the asset token (USTB) in exchange for the liquidity token (USDC)
+   * @param amount The amount of the USDC token to exchange, in base units
+   */
+  function subscribe(uint256 amount) external {
+    uint256 USTBAmount = amount / USTBPrice;
+    IERC20(liquidity).safeTransferFrom(msg.sender, address(this), amount);
+    IERC20(asset).safeTransfer(msg.sender, USTBAmount);
+  }
+
+  /**
+   * @notice Redeem the asset token (USTB) in exchange for the liquidity token (USDC)
+   * @param amount The amount of the USTB token to exchange, in base units
+   */
+  function redeem(uint256 amount) external {
+    uint256 USDCAmount = amount * USTBPrice;
+    IERC20(asset).safeTransfer(msg.sender, amount);
+    IERC20(liquidity).safeTransferFrom(msg.sender, address(this), USDCAmount);
+  }
+
+  /**
+   * @notice Set the price of USTB, amount of USDC for 1 USTB. Accounts for decimal mismatch.
+   * @param newPrice The new price of USTB
+   */
+  function setUSTBPrice(uint256 newPrice) public {
+    USTBPrice = newPrice;
+  }
+}