coverage: Add a test for async blocks

We have coverage tests that use async functions, but none that use async
blocks.

tests/coverage/async_block.rs

@@ -0,0 +1,35 @@
+#![feature(coverage_attribute)]
+#![feature(noop_waker)]
+// edition: 2021
+
+fn main() {
+    for i in 0..16 {
+        let future = async {
+            if i >= 12 {
+                println!("big");
+            } else {
+                println!("small");
+            }
+        };
+        executor::block_on(future);
+    }
+}
+
+mod executor {
+    use core::future::Future;
+    use core::pin::pin;
+    use core::task::{Context, Poll, Waker};
+
+    #[coverage(off)]
+    pub fn block_on<F: Future>(mut future: F) -> F::Output {
+        let mut future = pin!(future);
+        let waker = Waker::noop();
+        let mut context = Context::from_waker(&waker);
+
+        loop {
+            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
+                break val;
+            }
+        }
+    }
+}