correct slope scaling and stabilize exchange rate test using mulWad and cleaned borrow setup

Author: ademolahh

test/unit/ZToken.t.sol

@@ -21,6 +21,8 @@ contract ZTokenTest is Test {
 
     uint256 private constant MINIMUM_AMOUNT = 1e18;
 
+    uint256 public constant SECONDS_PER_YEAR = 365 days;
+
     function setUp() public {
         IZToken.MarketConfig memory config = IZToken.MarketConfig({
             underlyingToken: underlyingToken,
@@ -39,9 +41,9 @@ contract ZTokenTest is Test {
 
         assertEq(zToken.UTILIZATION_THRESHOLD(), UTILIZATION_THRESHOLD);
 
-        assertEq(zToken.SLOPE_BEFORE_KINK(), SLOPE_BEFORE_KINK);
+        assertEq(zToken.SLOPE_BEFORE_KINK(), SLOPE_BEFORE_KINK / SECONDS_PER_YEAR);
 
-        assertEq(zToken.SLOPE_AFTER_KINK(), SLOPE_AFTER_KINK);
+        assertEq(zToken.SLOPE_AFTER_KINK(), SLOPE_AFTER_KINK / SECONDS_PER_YEAR);
 
         assertEq(zToken.BASE_BORROW_RATE(), BASE_BORROW_RATE);
 
@@ -55,13 +57,11 @@ contract ZTokenTest is Test {
     }
 
     function test_GetExchangeRate_WhenATokenHasBeenBorrowed(uint256 _cash, uint256 _borrow) external {
-        zToken.setTotalBorrow(_borrow);
-
-        _cash = bound(_cash, MINIMUM_AMOUNT, type(uint128).max);
+        _cash = bound(_cash, MINIMUM_AMOUNT, type(uint96).max);
         _borrow = bound(_borrow, _cash / 10, _cash);
 
         zToken.setTotalCash(_cash);
-        zToken.setTotalSupply(_cash.mulDiv(zToken.INITIAL_EXCHANGE_RATE(), 1e18));
+        zToken.setTotalSupply(_cash.mulWad(zToken.INITIAL_EXCHANGE_RATE()));
 
         zToken.setTotalBorrow(_borrow);
         zToken.setTotalReserves(0);