util: Add TokioContext future (#2791)

Co-authored-by: Lucio Franco <[email protected]>

tokio-util/tests/context.rs

@@ -0,0 +1,28 @@
+#![warn(rust_2018_idioms)]
+
+use tokio::runtime::Builder;
+use tokio::time::*;
+use tokio_util::context::HandleExt;
+
+#[test]
+fn tokio_context_with_another_runtime() {
+    let mut rt1 = Builder::new()
+        .threaded_scheduler()
+        .core_threads(1)
+        // no timer!
+        .build()
+        .unwrap();
+    let rt2 = Builder::new()
+        .threaded_scheduler()
+        .core_threads(1)
+        .enable_all()
+        .build()
+        .unwrap();
+
+    // Without the `HandleExt.wrap()` there would be a panic because there is
+    // no timer running, since it would be referencing runtime r1.
+    let _ = rt1.block_on(
+        rt2.handle()
+            .wrap(async move { delay_for(Duration::from_millis(2)).await }),
+    );
+}