Auto merge of #76368 - ayushmishra2005:move_str_contact_library, r=jyn514

Added str tests in library

Added str tests in library  as a part of #76268

r? @matklad

Author: bors

library/alloc/tests/string.rs

@@ -721,3 +721,11 @@ fn test_from_char() {
     let s: String = 'x'.into();
     assert_eq!(s, 'x'.to_string());
 }
+
+#[test]
+fn test_str_concat() {
+    let a: String = "hello".to_string();
+    let b: String = "world".to_string();
+    let s: String = format!("{}{}", a, b);
+    assert_eq!(s.as_bytes()[9], 'd' as u8);
+}