Add test for string_search in C11

tests/test-mstring.c

@@ -807,6 +807,9 @@ static void test_M_LET(void)
         string_set_str(s3, "world");
         string_cats(s1, s2, " ", s3, "!");
         assert (string_cmp_str (s1, "Hello world!") == 0);
+        assert (string_search(s1, "wor") == 6);
+        string_set_str(s3, "orl");
+        assert (string_search(s1, s3) == 7);
       }
     }
 #endif