coverage: Extract executor::block_on from several async coverage tests

By moving `block_on` to an auxiliary crate, we avoid having to keep a separate
copy of it in every async test.

(This also incorporates some small tweaks to the headers in `await_ready.rs`.)

tests/coverage/async.rs

@@ -6,6 +6,9 @@
 //@ edition: 2018
 //@ compile-flags: -Copt-level=1
 
+//@ aux-build: executor.rs
+extern crate executor;
+
 async fn c(x: u8) -> u8 {
     if x == 8 {
         1
@@ -95,21 +98,3 @@ fn main() {
     let _ = m(5);
     executor::block_on(future.as_mut());
 }
-
-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 mut context = Context::from_waker(Waker::noop());
-
-        loop {
-            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
-                break val;
-            }
-        }
-    }
-}