Add forge-std.

lib/forge-std/test/Vm.t.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.8.0 <0.9.0;
+
+import {Test} from "../src/Test.sol";
+import {Vm, VmSafe} from "../src/Vm.sol";
+
+// These tests ensure that functions are never accidentally removed from a Vm interface, or
+// inadvertently moved between Vm and VmSafe. These tests must be updated each time a function is
+// added to or removed from Vm or VmSafe.
+contract VmTest is Test {
+    function test_VmInterfaceId() public pure {
+        assertEq(type(Vm).interfaceId, bytes4(0xdb28dd7b), "Vm");
+    }
+
+    function test_VmSafeInterfaceId() public pure {
+        assertEq(type(VmSafe).interfaceId, bytes4(0xe76e7868), "VmSafe");
+    }
+}