Fix ignoring test under miri

The `async_functions` module is not even compiled-but-ignored with miri, it is
completely `cfg`ed off. Therefore we ahve to do the same with this test that
imports stuff from that module.

tests/all/pooling_allocator.rs

@@ -927,7 +927,7 @@ fn total_tables_limit() -> Result<()> {
 }
 
 #[tokio::test]
-#[cfg_attr(miri, ignore)]
+#[cfg(not(miri)]
 async fn total_stacks_limit() -> Result<()> {
     use super::async_functions::PollOnce;