add test for evaluation errors

tests/assets/flake.nix

@@ -7,5 +7,7 @@
     in
     {
       hydraJobs = import ./ci.nix { inherit pkgs; };
+
+      legacyPackages.x86_64-linux.brokenPackage = throw "this is an evaluation error";
     };
 }

tests/test_eval.py

@@ -68,3 +68,16 @@ def test_expression() -> None:
 
     with open(TEST_ROOT.joinpath("assets/ci.nix"), "r") as ci_nix:
         common_test(["-E", ci_nix.read()])
+
+def test_eval_error() -> None:
+    with TemporaryDirectory() as tempdir:
+        cmd = [str(BIN), "--gc-roots-dir", tempdir, "--meta", "--flake", ".#legacyPackages.x86_64-linux"]
+        res = subprocess.run(
+            cmd,
+            cwd=TEST_ROOT.joinpath("assets"),
+            text=True,
+            stdout=subprocess.PIPE,
+        )
+        attrs = json.loads(res.stdout)
+        assert attrs["attr"] == "brokenPackage"
+        assert "this is an evaluation error" in  attrs["error"]