test: add failing test for linear relation

Author: mmaker

src/tests/test_validation_criteria.rs

@@ -131,6 +131,20 @@ mod instance_validation {
         let result = CanonicalLinearRelation::try_from(&relation);
         assert!(result.is_err());
     }
+
+    #[test]
+    fn without_witness() {
+        let B = G::generator();
+        let x = Scalar::from(42);
+
+        let mut linear_relation = LinearRelation::<G>::new();
+        let B_var = linear_relation.allocate_element();
+        let _X_var = linear_relation.allocate_eq(B_var * x + B_var * Scalar::from(3));
+
+        linear_relation.set_element(B_var, B);
+        let result = CanonicalLinearRelation::try_from(&linear_relation);
+        assert!(result.is_err());
+    }
 }
 
 #[cfg(test)]