Add integration tests

These initial tests validade basic usage with timed futures for:
- `System::block_on`;
- `Arbiter::new`;
- `Arbiter::stop`;
- `Arbiter::join`;

Signed-off-by: Jonathas-Conceicao <jadoliveira@inf.ufpel.edu.br>

actix-rt/tests/wait_spawned.rs

@@ -0,0 +1,63 @@
+use std::time::{Duration, Instant};
+
+#[test]
+fn await_for_timer() {
+    let time = Duration::from_secs(2);
+    let instant = Instant::now();
+    actix_rt::System::new("test_wait_timer").block_on(async move {
+        tokio::time::delay_for(time).await;
+    });
+    assert!(
+        instant.elapsed() >= time,
+        "Block on should poll awaited future to completion"
+    );
+}
+
+#[test]
+fn join_another_arbiter() {
+    let time = Duration::from_secs(2);
+    let instant = Instant::now();
+    actix_rt::System::new("test_join_another_arbiter").block_on(async move {
+        let mut arbiter = actix_rt::Arbiter::new();
+        arbiter.send(Box::pin(async move {
+            tokio::time::delay_for(time).await;
+            actix_rt::Arbiter::current().stop();
+        }));
+        arbiter.join().unwrap();
+    });
+    assert!(
+        instant.elapsed() >= time,
+        "Join on another arbiter should complete only when it calls stop"
+    );
+
+    let instant = Instant::now();
+    actix_rt::System::new("test_join_another_arbiter").block_on(async move {
+        let mut arbiter = actix_rt::Arbiter::new();
+        arbiter.exec_fn(move || {
+            actix_rt::spawn(async move {
+                tokio::time::delay_for(time).await;
+                actix_rt::Arbiter::current().stop();
+            });
+        });
+        arbiter.join().unwrap();
+    });
+    assert!(
+        instant.elapsed() >= time,
+        "Join on a arbiter that has used actix_rt::spawn should wait for said future"
+    );
+
+    let instant = Instant::now();
+    actix_rt::System::new("test_join_another_arbiter").block_on(async move {
+        let mut arbiter = actix_rt::Arbiter::new();
+        arbiter.send(Box::pin(async move {
+            tokio::time::delay_for(time).await;
+            actix_rt::Arbiter::current().stop();
+        }));
+        arbiter.stop();
+        arbiter.join().unwrap();
+    });
+    assert!(
+        instant.elapsed() < time,
+        "Premature stop of arbiter should conclude regardless of it's current state"
+    );
+}