Apply clippy suggestions

src/validators/posts.rs

@@ -41,7 +41,7 @@ mod tests {
     fn test_clean_content_empty() {
         let content = "  ";
         let result = clean_content(content);
-        assert!(!result.is_ok());
+        assert!(result.is_err());
     }
 
     #[test]

src/validators/users.rs

@@ -23,6 +23,6 @@ mod tests {
         let result_1 = validate_local_username("name_1");
         assert!(result_1.is_ok());
         let result_2 = validate_local_username("name&");
-        assert!(!result_2.is_ok());
+        assert!(result_2.is_err());
     }
 }