ascii: Improve test

tests/snippets/builtin_ascii.py

@@ -2,4 +2,6 @@ assert ascii('hello world') == "'hello world'"
 assert ascii('안녕 세상') == "'\\uc548\\ub155 \\uc138\\uc0c1'"
 assert ascii('안녕 RustPython') == "'\\uc548\\ub155 RustPython'"
 assert ascii(5) == '5'
-assert ascii(chr(0x10001)) == "'\\U00010001'"
+assert ascii(chr(0x10001)) == "'\\U00010001'"
+assert ascii(chr(0x9999)) == "'\\u9999'"
+assert ascii(chr(0x0A)) == "'\\n'"

vm/src/builtins.rs

@@ -66,7 +66,7 @@ fn builtin_ascii(obj: PyObjectRef, vm: &VirtualMachine) -> PyResult<String> {
         if c.is_ascii() {
             ascii.push(c);
         } else if (c as i64) < 0x10000 {
-            let hex = format!("\\u{:x}", c as i64);
+            let hex = format!("\\u{:04x}", c as i64);
             ascii.push_str(&hex);
         } else {
             let hex = format!("\\U{:08x}", c as i64);