autofree: add skipped string_plus_string_plus.v sample

vlib/v/tests/valgrind/string_plus_string_plus.v

@@ -0,0 +1,11 @@
+fn abcd() string {
+	a := 'abc'
+	b := 'def'
+	c := 'xyz'
+	d := '123'
+	return a + b + c + d
+}
+
+fn main() {
+	println(abcd())
+}

vlib/v/tests/valgrind/valgrind_test.v

@@ -28,6 +28,7 @@ const skip_valgrind_files = [
 	'vlib/v/tests/valgrind/fn_returning_string_param.v',
 	'vlib/v/tests/valgrind/fn_with_return_should_free_local_vars.v',
 	'vlib/v/tests/valgrind/option_simple.v',
+	'vlib/v/tests/valgrind/string_plus_string_plus.v',
 ]
 
 fn vprintln(s string) {