Move test-cases in string.rs

Author: Ayush Kumar Mishra

library/alloc/tests/str.rs

@@ -1921,11 +1921,3 @@ fn different_str_pattern_forwarding_lifetimes() {
 
     foo::<&str>("x");
 }
-
-#[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);
-}

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);
+}