Fix embassy-futures test failure

Running `cargo test` in embassy-futures was failing.  The `no_run` tag
on this doc example caused it to still try and compile this example,
just not run it, and compilation failed.  This updates the example so
that it can successfully compile and run.

embassy-futures/src/yield_now.rs

@@ -9,10 +9,16 @@ use core::task::{Context, Poll};
 /// hold, while still allowing other tasks to run concurrently (not monopolizing
 /// the executor thread).
 ///
-/// ```rust,no_run
+/// ```rust
+/// # use embassy_futures::{block_on, yield_now};
+/// # async fn test_fn() {
+/// # let mut iter_count: u32 = 0;
+/// # let mut some_condition = || { iter_count += 1; iter_count > 10 };
 /// while !some_condition() {
 ///     yield_now().await;
 /// }
+/// # }
+/// # block_on(test_fn());
 /// ```
 ///
 /// The downside is this will spin in a busy loop, using 100% of the CPU, while