actix-rt: Add Arbiter::is_running helper and fix System::is_set doc

`Arbiter::is_running` can be used to check if the current even-loop is currently
running; which should also work after the system has stopped. `System::is_set`
was updated to reflect what it actually does, it tells if the event loop has
started, which alone can't tell if it has stopped.

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

actix-rt/tests/integration_tests.rs

@@ -1,5 +1,19 @@
 use std::time::{Duration, Instant};
 
+#[test]
+fn start_and_stop() {
+    actix_rt::System::new("start_and_stop").block_on(async move {
+        assert!(
+            actix_rt::Arbiter::is_running(),
+            "System doesn't seem to have started"
+        );
+    });
+    assert!(
+        !actix_rt::Arbiter::is_running(),
+        "System doesn't seem to have stopped"
+    );
+}
+
 #[test]
 fn await_for_timer() {
     let time = Duration::from_secs(2);