KCL: Improve error messages for var referenced in own definition (#7374)

Jon pointed out that my new error message wasn't showing up in some
cases, and it should store/restore the previous var being defined.

rust/kcl-lib/src/simulation_tests.rs

@@ -3525,3 +3525,24 @@ mod ascription_unknown_type {
         super::execute(TEST_NAME, true).await
     }
 }
+mod var_ref_in_own_def_decl {
+    const TEST_NAME: &str = "var_ref_in_own_def_decl";
+
+    /// Test parsing KCL.
+    #[test]
+    fn parse() {
+        super::parse(TEST_NAME)
+    }
+
+    /// Test that parsing and unparsing KCL produces the original KCL input.
+    #[tokio::test(flavor = "multi_thread")]
+    async fn unparse() {
+        super::unparse(TEST_NAME).await
+    }
+
+    /// Test that KCL is executed correctly.
+    #[tokio::test(flavor = "multi_thread")]
+    async fn kcl_test_execute() {
+        super::execute(TEST_NAME, true).await
+    }
+}