testsuite/rust: add a xfail test case ...

... to show case the timeout system is working

gcc/testsuite/rust/compile/xfail/macro_return.rs

@@ -0,0 +1,10 @@
+// { dg-excess-errors "...." { xfail *-*-*-* } }
+
+macro_rules! add {
+    ($a:expr) => { $a };
+    ($a:expr, $($b:expr),+) => { $a + add!($($b),*) }
+}
+
+fn main() -> i32 {
+    add!(add!(1, 2))
+}