feat(content): hidden test cases

backend/data/questions/valid-palindrome.yaml

@@ -26,6 +26,14 @@ test_cases:
       expected: false
     - input: { s: "Aa" }
       expected: true
+    - input: { s: "" }
+      expected: true
+    - input: { s: ".,!" }
+      expected: true
+    - input: { s: "0P" }
+      expected: false
+    - input: { s: "Was it a car or a cat I saw?" }
+      expected: true
 
 description: |
   A phrase is a **palindrome** if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.