Add test for string AddAssign

src/libcollectionstest/string.rs

@@ -192,6 +192,17 @@ fn test_push_str() {
     assert_eq!(&s[0..], "abcประเทศไทย中华Việt Nam");
 }
 
+#[test]
+fn test_add_assign() {
+    let mut s = String::new();
+    s += "";
+    assert_eq!(s.as_str(), "");
+    s += "abc";
+    assert_eq!(s.as_str(), "abc");
+    s += "ประเทศไทย中华Việt Nam";
+    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam");
+}
+
 #[test]
 fn test_push() {
     let mut data = String::from("ประเทศไทย中");