Added hevm.sol

src/hevm.sol

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+pragma solidity >=0.4.23;
+
+abstract contract Hevm {
+   function warp(uint256 x) external virtual;
+   function roll(uint256 x) external virtual;
+   function store(address c,bytes32 loc,bytes32 val) external virtual;
+   function load(address c,bytes32 loc) external virtual returns (bytes32 val);
+   function sign(uint256 sk,bytes32 digest) external virtual returns (uint8 v, bytes32 r, bytes32 s);
+   function addr(uint256 sk) external virtual returns (address addr);
+   function ffi(string[] calldata) external virtual returns (bytes memory);
+}

src/test.sol

@@ -15,6 +15,8 @@
 
 pragma solidity >=0.4.23;
 
+import "./hevm.sol";
+
 contract DSTest {
     event log                    (string);
     event logs                   (bytes);
@@ -40,6 +42,8 @@ contract DSTest {
 
     address constant HEVM_ADDRESS =
         address(bytes20(uint160(uint256(keccak256('hevm cheat code')))));
+
+    Hevm public hevm = Hevm(HEVM_ADDRESS);
 
     modifier mayRevert() { _; }
     modifier testopts(string memory) { _; }