test: add early fail() test

tests/expected/all.json

@@ -777,6 +777,17 @@
                 "num_bounded_loops": null
             }
         ],
+        "test/Foundry.t.sol:EarlyFailTest": [
+            {
+                "name": "check_early_fail_cheatcode(uint256)",
+                "exitcode": 1,
+                "num_models": 1,
+                "models": null,
+                "num_paths": null,
+                "time": null,
+                "num_bounded_loops": null
+            }
+        ],
         "test/Foundry.t.sol:FoundryTest": [
             {
                 "name": "check_assume(uint256)",

tests/regression/test/Foundry.t.sol

@@ -21,6 +21,22 @@ contract DeepFailer is Test {
     }
 }
 
+contract EarlyFailTest is Test {
+    function do_fail() external {
+        fail();
+    }
+
+    function check_early_fail_cheatcode(uint x) public {
+        address(this).call(abi.encodeWithSelector(this.do_fail.selector, ""));
+
+        // this shouldn't be reached due to the early fail() semantics.
+        // if this assertion is executed, two counterexamples will be generated:
+        // - counterexample caused by fail(): x > 0
+        // - counterexample caused by assert(x > 0): x == 0
+        assert(x > 0);
+    }
+}
+
 contract FoundryTest is Test {
     /* TODO: support checkFail prefix
     function checkFail() public {